Question: how to differentiate when 'siguro' is used as 'maybe'

“Siguro (maybe)” was most likely a loanword from the Spanish “seguro (sure)” - strangely though, it means the opposite in Filipino. It’s the same case with the Spanish “demasiado (very)” and the Filipino “ ’di masyado (not so much)”.“Siguro” only means “maybe” in Filipino. “Sigurado” is what we use for “sure” and its closest match in Spanish is “asegurado (secured)”.What you were worried about as your possible misinterpretation of its meaning is actually its CORRECT interpretation. SIGURO siya AY lagpas/lampas (na) ng sesenta (anyos). = MAYBE he is (already) over sixty (years old). - You may omit the “na” but we usually add it there. Your sentence may also sound more natural with it there. The “anyos” is unnecessary if based on the context it is the age that is being referred to. Without context, 60 could mean kilos, a speedometer measure, or any other measurement that might make sense. (“SIGURO lagpás/lampás (na) siya ng sesenta (anyos)” is how to say it without using “ay”).SIGURADOng siya AY lagpas/lampas (na) ng sesenta (anyos) = SIGURADOng lagpas/lampas (na) siya ng sesenta (anyos). = SURELY he’s (already) over sixty (years old). - SIGURADO ako na lagpas/lampas (na) siya ng sesenta (anyos). = I am SURE he’s (already) over sixty (years old). Hindi ako SIGURADO pero SIGURO may sesenta na siya. = I am not SURE but MAYBE he is already in his sixties. We would say "He must be over sixty" as “MALAMÁNG lagpas/lampas (na) siya ng sesenta (anyos). [MOST LIKELY he is (already) over sixty (years old)]”. - We don’t have the right word for “must” that would correspond to the sense given in the sentence. “Most likely” is the closest match. For “must” we usually use “kailangan” but it would translate to “needs to be” or “has to be”. We can also use “dapat” and make it translate to “must” if we’d extend the sentence to add context to it. For example, “Dapat lagpas/lampas na siya ng sesenta kasi si Eisenhower pa ang presidente ng US nang ipinanganák siya”. = He must be over 60 because Eisenhower was still the US president when he was born. 🙂

For fun I compared 4 dictionaries: TDC, UPDF, KWF and Leo. All 4 included definitions like "perhaps, maybe, probably" (adverbs) but only TDC defined "siguro" as "sure" and Leo explicitly stated that "siguro" does not mean "sure, certain" (adjectives).TDC: [adverb] maybe •sure •perhaps •probably
UPDF: pnb (adverb) : marahil
KWF: Pang-abay (adverb) : Marahil; malamang; may posibilidad.
Leo : adv. perhaps.My take from the English side. I try to keep in mind that English words have different meanings too and that includes "sure" and "must be". In English they can be used to express more of an opinion or assertion rather than a certainty or imperative.
"Surely you jest" or "You must be joking" doesn't mean that you are definitely jesting but that is how it seems - "You are probable jesting, right?".
"He must be 60 if he's a day" wouldn't mean he is definitely 60 but it seems likely by the way he looks and acts (or by some other deduction).

Don't use 'siguro' to say 'sure'. It means 'maybe' solely maybe pertaining to uncertainty. Both of your examples are correct tho. Parehong ginagamit and maybe. Same lang din meaning na 'siguro'. However 'Sigurado' means 'Surely'. For example: "Sigurado nag lalaro sya bc.game kaya hindi sya nag rereply". Meaning Surely he's playing. But if you say "Siguro nag lalaro" it means He must be playing or he may be playing. Hope this helps and doesn't confuse you.