acho que stay = ficar; keep = permanecer - os dois servem para pessoas e um tem o sentido mais forte que o outro. Os exemplos estão no longman.
Don’t keep us in suspense any longer!
The police put up barriers to keep the crowds back. If I were you, I’d keep away from that area at night. a sign saying ‘Danger: Keep Out’ The little boy kept close to his mother.
This is the imperative tense: Stay! Go away! Come here now! The subject is not usually mentioned in English, and it's not used in Portuguese either. Portuguese uses three forms depending on whom you are addressing. I learned from my grammar book to use the second form (fique), but have since learned that some Brazilians consider that a bit strong and prefer the "tu" form: fica.
The present tense in Portuguese conjugates according to first, second and third persons, singular and plural. www.conjuga-me.net is a good site to see the various conjugations in Portuguese.
Thanks for your in depth reply (and the website). From what I can see, the Tu form of the imperative tense/mood is always the same spelling as the Você form of the present tense? Is that correct? Present: "Você anda com tubarões?" Imperative (você form): "Tubarões! Ande!" Imperative (tu form): "Tubarões! Anda!" So basically, if we always use the Tu form, nothing really changes?
I haven't verified that all the verbs in the "ar" conjugation follow that pattern, but it may be so.
Checking on a verb in the "er" conjugation: comer, we have "ele come" (present tense) with come (tu), coma (você) e comam (vocês). The same pattern is with "correr".
"Partir": ele parte (present) and parte (tu), parta (você) e partam (vocês). Same pattern
There are, of course, the irregular verbs. Here's one. Ter. Ele tem (present). Imperative: tem (tu), tenha (você), tenham (vocês). In this case, the most common form that you hear is "tenha".
I guess there is a pattern. ;)