Questions & Answers about Želiš li večerati sa mnom?
Why is it sa mnom and not just s mnom?
The preposition s (with) expands to sa to make pronunciation easier. This always happens before words starting with certain tricky consonants (like s, z, š, ž) and specifically before mnom. Therefore, "with me" will always be written and spoken as sa mnom.
Where does the word mnom come from?
Mnom is the instrumental case form of the pronoun ja (I). Whenever you talk about doing something "with" someone, you use the preposition s or sa followed by the instrumental case.
Why is the question word li placed right after Želiš?
The question particle li is an enclitic, which is a "weak" word that cannot stand on its own or start a sentence. Grammatically, it must always take the second position in a sentence, which is why it tucks in immediately after the first word, Želiš.
Could I use the noun večeru (dinner) instead of večerati here?