Learn Russian: Профессионал постоянно совершенствует свои навыки. - A professional constantly improves his skills.

The infinitive is совершенствовать (to improve or perfect). Verbs ending in -овать drop the -ова- and replace it with -у- in the present tense before adding the regular endings. Therefore, the stem becomes совершенству- plus the third-person singular ending -ет.

Is совершенствует imperfective, and what is its perfective counterpart?

Yes, совершенствовать is imperfective, which is required here because the action happens постоянно (constantly). Its perfective pair is усовершенствовать, which you would use to describe a one-time completed improvement.

Why must we use свои here instead of его for 'his'?

Because the skills belong to the subject of the sentence (Профессионал). Whenever the possessor and the subject are the same person, Russian requires the reflexive possessive pronoun свой. If you used его here, it would mean the professional is improving some other man's skills.

How does навык differ from other words for 'skill' or 'ability', such as умение?

Навык specifically refers to an automatic, practical skill or habit developed through physical repetition and practice, such as typing or operating machinery. Умение is a broader term for a learned ability or the theoretical know-how to accomplish a task.