Learn Russian: Он внимательный студент.

In Russian, the verb "to be" is almost always left out in the present tense. Instead of saying "He is a student," you simply string the subject and the description together: "He an attentive student" (Он внимательный студент).

The current lesson is about adverbs. Why do we use внимательный here instead of an adverb like внимательно?

Adverbs describe actions (verbs), while adjectives describe nouns (people, places, or things). Because this sentence describes the noun студент (student), we use the adjective внимательный (attentive). If the sentence described how he does an action, we would use the adverb: Он слушает внимательно (He listens attentively).

Why does внимательный end in -ый?

Russian adjectives must match the gender of the noun they describe. Because студент is a masculine noun, the adjective takes the standard masculine dictionary ending -ый. If we were talking about a female student, the ending would change to match the feminine gender: Она внимательная студентка.

Is the word order strictly Subject + Adjective + Noun here?

Russian word order is flexible, but for a simple, neutral description like this, the standard order is exactly like English: Subject (Он) + Adjective (внимательный) + Noun (студент). Moving the adjective after the noun is possible but usually sounds poetic or adds heavy emotional emphasis.