Learn Hardcore Ukrainian: Ввечері я також читаю книгу.

Both Ввечері and Увечері are correct and mean the same thing.
• Ввечері is a shortened adverbial form.
• Увечері uses the preposition у plus the noun вечері.
Stylistically they’re interchangeable, though some speakers prefer one over the other.

What does також mean? How is it different from теж or і?

також means “also” or “too.”
• теж also means “too” but usually follows the word it emphasizes (e.g., я теж читаю).
• і means “and” or “too” in colloquial speech but is less formal than також or теж.

Is the subject я required? Could you omit it?

No, я (“I”) is not required because Ukrainian verbs carry person endings.
• Читаю книгу already means “(I) read/am reading a book.”
• Including я adds emphasis or clarity (e.g., contrasting with someone else).

What case and number is книгу in?

книгу is singular feminine in the accusative case.
• It’s the direct object of читаю, answering “what?” (a book).

What tense and aspect is читаю? Does it mean “I read” or “I am reading”?

читаю is present tense, imperfective aspect.
• It can mean “I am reading (right now)” or “I read (habitually/every evening).”
• Context (time expressions, tone) tells you which.

Could I change the word order without changing the meaning?

Yes. Ukrainian word order is flexible. For example:
• Я також ввечері читаю книгу.
• Читаю книгу я також ввечері.
All versions express the same idea, though the focus or emphasis may shift.