Learn Turkish: Siz alfabeyi çok çabuk öğrenmişsiniz. - Apparently you learned the alphabet very quickly.

The suffix -miş is used for the evidential past tense, which is a major topic at the A2 level. Instead of a direct statement of fact, it expresses that the speaker realized or deduced this after the fact. The speaker didn't witness you learning, but sees the result and infers it, hence the translation 'Apparently, you learned...'. The -siniz part is the formal or plural 'you' ending.

Why is there a -yi at the end of alfabeyi?

Alfabe means alphabet. The -i suffix is the definite accusative case marker, and the -y- is a buffer letter to separate two vowels. You use this case when a specific direct object receives the action of the verb. Here, you are learning a specific thing ('the' alphabet), so it takes the accusative marker.

Does çabuk mean 'quick' or 'quickly'?

It actually functions as both! In Turkish, you usually do not need to add a special suffix to turn an adjective into an adverb (like '-ly' in English). You just place the standard adjective before the verb. Here, placed before öğrenmişsiniz, the adjective çabuk naturally acts as the adverb 'quickly'.

Is the word siz strictly necessary at the beginning of the sentence?

No, you could simply say Alfabeyi çok çabuk öğrenmişsiniz. Because Turkish drops pronouns easily, the -siniz ending on the verb already tells us exactly who did the learning. Including siz at the start is optional and just adds a little bit of emphasis to the subject.