Learn Italian: Lui era un buono studente che studiava molto. - He was a good student who studied a lot.

The adjective buono changes its form depending on the sound of the masculine noun that follows it. Because studente starts with an s and another consonant (an impure s), buono keeps its full form. If the noun started with most other consonants or a vowel, it would shorten to buon (like un buon libro).

Why is the article un instead of uno? Do words starting with st not take uno?

The indefinite article always matches the word directly after it, not the noun at the end of the phrase. Since the word right after the article is buono, which starts with a normal consonant, we use un. If the adjective was not there, it would indeed be uno studente.

Why do we use the imperfect tense verbs era and studiava here?

The imperfect tense is used for descriptions and ongoing or repeated actions in the past. Here, era describes a past state (what kind of student he was), and studiava describes a habitual action (he used to study a lot).

Why is who translated as che and not chi?