@Bennie329
The response of @jkos about the use of “may” is correct. I will just elaborate on it and also address your other concerns below.
MAY AKO
“May AKO isang kape lang” is an ambiguous Tagalog sentence. Its literal translation is “There is I one coffee only”. So, unless there is context to base your sentence on, it would be almost impossible to understand.
Reasons: “MAY” can be used before a common noun, but NOT BEFORE a subject pronoun unless you are referring to the pronoun as just a word independent of its “I” meaning. For example,
“May AKO sa pangungusap na ito” would be understood correctly as “There is an “AKO” in this sentence. - The word "ako" remains as "ako", i.e., it is not translated into "I".
We can also add AKO (or another pronoun) after MAY in a sentence like this: “Hindi kami pareho, may SILA at may AKO”. It translates to “We are not the same, there’s THEM and there’s ME. In this case, though, the pronouns are no longer subject/actor, but object/non-actor pronouns.
Instead of using "MAY", you may want to use “MAYROON (or, "meron")” because it can be used before a common noun or a subject pronoun.
MAYROON AKOng isang kape lang. = I only have one coffee.
However, notice that the linker “na” gets into the picture as the “ng” in “AKOng”. The sentence is OK, but it would sound more natural if stated as, “Mayroon lang akong isang kape” instead.
MAY mga langgam sa kape ko. = There are ants in my coffee. (“Langgam (ant)” is a common noun.)
MAYROONg mga langgam sa kape ko. = There are ants in my coffee.
So, “may” directly indicates the existence of the word that immediately follows it – as what @jkos indicated as "May [noun] [pronoun]". And that’s why “MAY AKO isang kape lang” translates to “THERE IS I one coffee only” because the following word, “ako” will automatically be translated to what it means, i.e., “I”.
“Mayroon”, on the other hand, would require the identification of what it is that it describes as existing – thus, the need for the “na” to link “mayroon” to that thing that exists. “MAYROON akoNG ISANG KAPE lang” translates to “I HAVE ONE COFFEE only” because it is the word(s) that follows the linker “na” that identifies what exists/you have.
“Mayroon” may be linked directly to the word that follows it except when that word is a pronoun. In such cases, the pronoun immediately follows “mayroon”, and the linker “na” ends up between the pronoun and the word that “mayroon” refers to.
Mayroong kotse SI MARY <> Mayroon SIYANG kotse.
Mayroong kausap SINA TOM AT JERRY <> Mayroon SILANG kausap.
When speaking, we may also place the proper name(s) between “mayroon” and the word(s) it is linked to. For example, it is all right for us to say “Mayroon si Maryng kotse”. However, since the appended “ng” may create confusion about the true proper name in the written form, e.g., John becomes Johng or Helen becomes Heleng, we, therefore, normally place the proper name after the linked word instead. Also, technically, it would be correct to say “Mayroon si Mary na kotse”, but it would sound so awkward to us that we just don’t do it. So, in its written form, only "Mayroong kotse si Mary" is acceptable.
Regarding the Pinay’s sentence, “May isang tasang kape lang AKO” (or “Mayroong isang tasang kape lang AKO”), it is the same as saying “AKO ay may/mayroong isang tasang kape lang”. It translates to “I have one cup of coffee only/I only have one cup of coffee”. The “ako” pronoun functions as the subject/actor of the sentence. So, it may either be at the end or at the beginning of the sentence.
Note: If my translations above sound awkward, it's because I took into account all the Tagalog words you used in your conversation. I am not sure if those are the ideas that both of you meant to convey to each other though. Do let me know if you meant something else.
PLACEMENT OF AKO
Should “ako”, or any other subject pronoun, be placed at the beginning or the end of a sentence?
It all depends on how you want to state your sentence. As I mentioned above, “May isang tasang kape lang AKO (1)” is just the same as saying “AKO ay may isang tasang kape lang (2)”, hence, both ways are correct.
Generally speaking, Tagalog sentences may begin with the subject or the predicate or the verb.
Sentence (1) above started with the predicate, while sentence (2) started with the subject/actor. “May/Mayroon” is “there is”, therefore, the actual verb in the sentence is “to be” even if we may translate it as “to have” to adapt it better into English. Since there is no “to be” verb in Tagalog, that sentence can’t be started with a verb.
Your sentence “Umiinom AKO isang tasang kape na”, starts with a verb, but the sentence is not correct. Stating it correctly could either be:
1) Umiinom NA AKO NG isang tasang kape = I AM ALREADY drinking a cup of coffee. (In Tagalog, the “na (already)” has to immediately follow the verb. Also, in Tagalog, the verb needs a preposition to link it to its object, that’s why we need a “ng (of)” before “isang tasang kape”, which literally translates to “drinking OF one cup of coffee”.)
OR
2) Umiinom AKO NG isang tasang kape. = I am drinking a cup of coffee.
Using sentence (2), it may be expressed starting with the actor/subject: AKO ay umiinom ng isang tasang kape. = I am drinking/drink a cup of coffee.
The sentence may also begin with the predicate, but since what follows the verb is its object, it means that the verb focus will then have to shift to the object (non-actor) and as a result, a non-actor focus verb has to be used. (“Umiinom” is an actor-focus verb and its non-actor focus verb counterpart is “Iniinom”.)
Iniinom KO ang isang tasang kape. = I am drinking/drink a/the cup of coffee. – Here, “ko” is only the actor and no longer the subject of the sentence. It immediately follows the verb at the start of the sentence.
The sentence may also be stated as:
“Ang isang tasang cape ay iniinom ko” = I am drinking/drink a/the cup of coffee, where it shows that the subject of the sentence has become the cup of coffee. Also note that the actor pronoun is at the end of the sentence this time.
(Note: When a non-actor focus verb is used, the object of the verb has to be either in its entirety or specific or both. It is for that reason that there is an option to use “a” or “the” before the “cup of coffee”. Using “a” would mean that it’s a cup of coffee and not any other type of drink, and using “the” would mean that it’s this specific cup of coffee and not any other cup of coffee.)
So, the placement of the subject pronoun would depend on how you’d state a Tagalog sentence. In the case of short or simple Tagalog sentences, we would normally start it with the verb and, therefore, the actor pronoun would be close to it. In longer sentences, and in particular, those that have direct and indirect objects, the verb might not be used to start the sentence and, as such, the actor pronoun may end up elsewhere in it. Identifying the actor, the direct and indirect objects, the subject, and the predicate can help you sort things out.