あなたのペンはどれですか。

you (referring to someone of equal or lower status); dear (what a wife calls a husband)

indicates possessive; nominalizes verbs and adjectives; substitutes for "ga" in subordinate phrases; (at sentence-end, falling tone) indicates a confident conclusion; (at sentence-end) indicates emotional emphasis; (at sentence-end, rising tone) indicates question

pen

topic marker particle; indicates contrast with another option (stated or unstated); adds emphasis

which (of three or more); whichever; any; well; now; let me see

be; is

indicates a question (sometimes rhetorical); or; whether or not; some- (e.g. something, someone); indicates doubt, uncertainty, etc.; emphatic prefix; suffix forming adjectives or adverbs; in that way