I've never studied grammar.
I want to know if something is a sentence, but don't know enough grammar to describe it and so ask a general question. Only, I think, create analogous sentences (I think?).
Is that OK?
I you're having trouble learning English, then please look up the English Learner's Stack Exchange Site, https://ell.stackexchange.com/.
If you want to know if your question is on topic, you need to be more specific in your question here. Strictly speaking asking if a particular collection of words is a sentence is off topic.
So if your question is "Is "A green baby" a sentence?", then that is off topic, as the definition of a sentence is something easily looked up.
But if your question leaves some kind of ambiguity in the definition of a collection of words as to whether they form a sentence then it's probably on topic.
It might also be worth your time looking up https://linguistics.stackexchange.com/, as they might have a similar type of question.