Learn Arabic (Modern Standard): يا زميلتي، لم تطبعي الملاحظات. - My colleague f, you did not print the notes.

Because of the word لم (did not) right before it. لم changes the verb into the Jussive mood. For a 'you (feminine singular)' verb, the Jussive rule is to drop the final ن. So the normal present tense تطبعين (you print) loses its ن and becomes لم تطبعي (you did not print).

The English translation says 'did not print' (past tense), but isn't تطبعي a present tense verb?

Yes! In Arabic, the particle لم is used with a present tense verb, but it changes the meaning to the past tense. So لم + present tense is the standard way to say 'did not' do something.

What is the purpose of يا at the beginning of the sentence?

يا is a calling word used when addressing someone directly. While English speakers just say 'My colleague', Arabic speakers naturally use يا before the person's name or title to get their attention, making it يا زميلتي.

How is the word for notes, الملاحظات, formed from the singular?

The singular word is ملاحظة (note). To make it plural, we remove the feminine ending ة (taa marboota) and add the regular feminine plural ending ـات (aat), giving us ملاحظات.