Learn Italian: Da ieri non fai altro che piagnucolare. - You've done nothing but whine since yesterday.

It expresses doing nothing but a specific action. You conjugate fare to match the subject (here, fai for the informal you), keep altro che invariable, and follow it immediately with an infinitive verb like piagnucolare.

Why is the present tense fai used when the English translation says "You've done"?

In Italian, when an action begins in the past and continues into the present, you use the present tense along with the preposition da (since). Because the person started whining yesterday (da ieri) and is still doing it now, Italian requires the present tense, unlike English which uses the present perfect.

Is the verb piagnucolare related to piangere?

Yes, it is derived from piangere (to cry). The suffix -ucolare modifies the original verb to describe doing the action in a weak, repetitive, or annoying way, which perfectly translates to whining or sniveling.

Could the che in this sentence be considered a relative pronoun?

No, in the non fare altro che structure, che functions as a conjunction meaning "than" or "except" in a comparative sense. The literal translation is closer to "you do not do other than whine."