Question: What’s the difference between kausapin and nakipag-u

@Alexbradfield
Yes, they both mean to talk to someone.
"Makipag-usap" is an actor focus and "kausapin" is an object focus.Example:
Kinausap ko ang mga bata sa paaralan kahapon.
I talked to the children at school yesterday.
(You were the one who initiated the talk)Nakipag-usap ako sa mga bata sa paaralan kahapon.
I had a conversation with the children at school yesterday.
(Either you or the children started the conversation)

@learntagalog so, I noticed you used the sa marker in the actor focus sentence “sa mga bata” not “ng mga bata”
But you didn’t in the first...? Too complicated to explain?

@Alexbradfield **Actor Focus Verbs use Ang phrase and Sa phrase Markers***
Nakipag-usap ako sa mga bata sa paaralan kahapon.
I had a conversation with the children at school yesterday.
"Ako/I" is the actor/doer/focus and "ang mga bata/the children" is the receiver/benefactor of the action.
VS
Nakipag-usap ang mga bata sa akin sa paaralan kahapon.
The children had a conversation with me at school yesterday.
"Ang mga bata/The children" is the actor/doer/focus and " ako/me" is the receiver /benefactor of the action.***In Object focus Verbs, "ang" marks the subject matter, "SA" marks the location and "Ng" marks the object.
Kinausap ko ang mga bata sa paaralan kahapon.
I talked to the children at school yesterday.
(I was the one (the doer) who initiated the talk)
"The children/Ang mga bata" were the focus and the receiver of the action.
VS
Kinausap ako ng mga bata sa paaralan kahapon.
The children talked to me at school yesterday.
(The children were (the doer/object ) the one who initiated the talk.)
"Me/ako" is now the focus and the receiver of the action

Focus explanations confused me for so long...FOCUS always felt like abstract grammar to me...soInstead of reading the writing here...imagine yourself listening to a speaker of the words.THENThink of the speaker as indicating who/what has his emphasis so you know where to draw your attention. In the verb he will indicate his emphasis (=focus) and direct your attention to the ACTOR on the verb or the RECEIVER of the verbs action. He chooses his verb intentionally as if saying "notice the actor" or "notice the receiver" of the action. (The verb CODES both of your attention...Sobrang ASTIG).Its true in English the actor is the subject and the receiver of the action is the object. BUT BUT BUT...forget Subject Verb Object order...SVO ORDER is ONLY going to confuse you. Let English go...just flow with the Tagalog ANG MARKER (not ANG = THE...not a wrong gloss but just interferes with ANGs most important role of identifying the topical focus).Just hear my verb emphasizing (focusing) on either the ACTOR to the verb or the RECEIVER of the verb's action. And I will indicate/point/mark that emphasis/focus with an ANG Pronoun or an ANG noun phrase. All the NG, SA, MGA, NI, NINA markers are all non-emphasis markers. Native speakers are crafty...order is FLUID. We MARK so order can be fluid. Just hear the emphasis in the verbs focus...and wait for the ANG to identify that pronoun or noun phrase.So the 4 example sentences in order...Nakipag-usap (actor focus verb)
ako (ang pronoun is the focus)
sa mga bata (no "ANG mga bata" because we used the ang pronoun to mark the actor...we only have one focus in a well formed sentence so after you use ANG once as a pronoun or noun phrase...no more ANG markers in the sentence)
sa paaralan kahapon.Nakipag-usap (actor focus verb)
ang mga bata (now we use ang with mga bata as an ang noun phrase to mark the actor)
sa akin sa paaralan kahapon.ang phrase/ang pronoun MARKS the FOCUS/emphasis....ALWAYSSSS....exactly the same in RECEIVER of action (object verb) focus belowKinausap (receiver "object" focus verb)
ko (ng pronoun marks the non-focus actor)
ang mga bata (ang noun phrase marks the focus receiver "object")
sa paaralan kahapon.Kinausap (receiver "object" focus verb)
ako (ang pronoun is the focus receiver "object")
ng mga bata sa paaralan kahapon.(ng noun phrase is the non-focus actor)Do you find ACTOR vs RECEIVER indicated by ANG....as helpful?AS a second language learner of Tagalog, I find Subject ( = Actor) Verb Object (= Receiver ) ORDER just freezes my mind in English which is not necessary or helpful in thinking in Tagalog. Of course, for NATIVE Tagalog speakers, the OBJECT notion is helpful for thinking to English in ESL learning.For me ACTOR to verb or RECEIVER of verb action to draw my attention and then listen/look for the ANG noun phrase or ANG PRONOUN (ako not ko, ka not mo, siya not niya -- make sure you are clear on all your ANG pronouns as compared to NG pronouns).

Makipagusap focuses on the "action", while kausapin focuses on the "object"/receipient."Makipagusap ka" is complete sentence, while "Kausapin mo" isn't. The latter requires the object to be specified

Answer 1:
"Talk to"Some are already feeling ittalk toshould only be used for one-sided conversation - when a television host is talking to viewers, perhaps, or when a boss charges an employee. However, imagine that someone is asking to speak with you. Will you remain completely silent, assuming that the request is a silent hope you will not respond? Probably not, because of everyday conversation,talk tois understood to mean "to talk to someone."For example-The employee became curious when his boss asked him to speak to him in the conference room.My husband hates me when I interrupt while he is talking to me."Talk to"Some people also feel ittalk toshould be reserved for discussions between two or more participants. Unlike expressiontalk toan exchange is implicit in the understanding of the prepositionincluded. However, it is rarely necessary to make such a strict distinction.For example-The baby often talks to his teddy bear about his day.Brent was asked if he could talk to the celebrity about his recent film.

Answer 2:
There is no significant difference when two people talk, and they both speak.Some people claim thattalk toshould be used when it is only one person speaking, andtalk toshould be used when it is more than a two-sided discussion.In practice, however, many native speakers are interchangeable. Here are some examples.Can I talk to you in a minute? = Can I talk to you in a minute?
Brian was there, talking to Matt. = Brian was there, talking to Matt.
I talked to my boss and he said I could take a day off. = I talked to my boss and he said I could leave one day.

Tagalog versions: Sagot 1:
"Kausapin si"Ang ilan ay nakakaramdam nakausapin sidapat gamitin lamang para sa isang panig na pag-uusap-kapag ang isang host ng telebisyon ay nakikipag-usap sa mga manonood, marahil, o kapag sinisingil ng isang boss ang isang empleyado. Gayunpaman, isipin na may humihiling na makipag-usap sa iyo. Mananatili kang ganap na tahimik, sa pag-aakala na sa kahilingan ay isang walang imik na pag-asang hindi ka tumugon? Marahil hindi, dahil sa pang-araw-araw na pag-uusap,kausapin siay naiintindihan na nangangahulugang "makipag-usap sa isang tao."Halimbawa-Naging mausisa ang empleyado nang hilingin ng kanyang boss na kausapin siya sa silid ng kumperensya.Kinamumuhian ng asawa ko nang makagambala ako habang kausap niya ako."Makipag-usap sa"Nararamdaman din ng ilang taomakipag-usap sadapat na nakalaan para sa mga talakayan sa pagitan ng dalawa o higit pang mga kalahok. Hindi tulad ng expressionkausapin si,ang isang pakikipagpalitan ay implicit sa pag-unawa ng preposisyonkasama. Gayunpaman, bihirang kinakailangan na gumawa ng tulad ng isang mahigpit na pagkakaiba.Halimbawa-Ang sanggol ay madalas na nakikipag-usap sa kanyang teddy bear tungkol sa kanyang araw.Tinanong si Brent kung maaari niyang kausapin ang tanyag na tao tungkol sa kanyang kamakailang pelikula.

Sagot 2:
Walang mahalagang pagkakaiba kapag ang dalawang tao ay nakikipag-usap, at pareho silang nagsasalita.Ang ilang mga tao ay inaangkin iyonkausapin sidapat gamitin kapag ito ay isang tao lamang na nagsasalita, atmakipag-usap sadapat gamitin kapag ito ay higit pa sa isang talakayan ng dalawang panig.Gayunpaman, sa pagsasagawa, maraming mga katutubong nagsasalita ang parehong magkakapalit. Narito ang ilang mga halimbawa.Maaari ba akong makipag-usap sa iyo ng isang minuto? = Maaari ba akong makipag-usap sa iyo sa isang minuto?
Nandoon si Brian, nakikipag-usap kay Matt. = Nandoon si Brian, nakikipag-usap kay Matt.
Kinausap ko ang aking boss at sinabi niya na maaari akong mag-alis ng araw. = Nakipag-usap ako sa aking boss at sinabi niyang makakapag-alis ako ng isang araw.

@Alexbradfield
My own interpretation of this is that some verbs (let's say, the root) decide if ng or sa is used, or even both for different purposes. usap works analogous to English in that you don't say "to talk someone" but "to talk TO someone", so "someone" is the thing that is the "beneficiary" of the action. Akward attempt to explain (but see the example later, I think it's clearer)
The verbs that use the "object focus forms" can either have an "object focus" (for lack of a different word... in English, this would normally be the object of a sentence) or a "beneficiary/locative focus" (in English, this is often marked with a "to"). The former means that ANG is used to mark the "object", the latter means that ANG is used to mark the beneficiary (which is marked with SA in any other focus).Example
bigay, to give: Giving has both an object and a beneficiary, e.g. The teacher (actor) gives the book (object) to the child (beneficiary)
We might want to mark all three nouns, so there are at least three verbs.Nagbibigay ang guro ng libro sa bata (Magbigay : actor focus)
The teacher gives a book to a child.Ibinibigay ng guro ang libro sa bata (Ibigay: object focus)
A teacher gives the book to a child.Binibigyan ng guro ng libro ang bata (bigyan: beneficiary focus)
A teacher gives a book to the child.
(oops, just noticed that's quite an old question... but, since I've already written down all of this, I'll post it anyway in case any future reader is interested)

Kausapin is about talk > future tenseNakipag-usap > past tense of makipag-usap