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first of all, those sentences were all translated properly, yes. (well done, to whoever did that, if not you yourself)
(but อย่าลืมล้างมือก่อนกินข้าว = "Don't forget 'to' wash your hands before dinner" or "Don't forget washing your hands before dinner". just notifying ^^)
and about how to use it, it's pretty ascribable to English's grammar in the way how it's positioned.(not perfect-ably, though)
ครั้งก่อน - last time <- ก่อน is modifying ครั้ง(time) to tell us that it's 'last' time. (since grammar rule in Thai is to put any modifier after a noun, contrast with English grammar at this point)
ขอคิดดูก่อน - let me see <- literally, "let me (try to) think before (I give you any further answer)" it's real form is : ขอ(ลอง)คิดดูก่อน(ที่จะให้คำตอบ) (ดู comes from ลอง...ดู pattern)
เราเคยเจอกันมาก่อนรึเปล่า - have we 'met' each other before ? <- break down into:
we've met - เราเคยเจอ
each other - กัน (so it's not literally match with English at this point but I think you understand it anyway)
before - มาก่อน
? - รึเปล่า (one of question words to change any sentence into question)
see? pretty ascribable, isn't it? and the reason why there has to be 'มา' in this is because we're indicating that it's jobbing as perfect tense. it's grammatical pattern. or instead, it should be like this:
เรา(เคย)เจอกัน(มา)ก่อนหน้านี้รึเปล่า? - (have) we met each other before this?
before this - ก่อนหน้านี้ (before the time being)
