Ben mesajlara zamanında yanıt veriyorum.

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Breakdown of Ben mesajlara zamanında yanıt veriyorum.

• mesaj: the message
• -lara: to
• zamanında: on time

Questions & Answers about Ben mesajlara zamanında yanıt veriyorum.

Why does the sentence start with Ben? Is it required to include it every time?

In Turkish the verb ending -iyorum already shows you are speaking about “I.” Including Ben (I) is optional and usually adds emphasis or clarity. Without it you can simply say:
Mesajlara zamanında yanıt veriyorum.

What case is mesajlara, and why is it formed that way?

Mesajlara breaks down as:
mesaj (message) + -lar (plural) + -a (dative).
The dative case -a/-e marks the thing you are “giving an answer to.” So mesajlara means “to the messages.”

Why is the word zamanında used here, and what do its endings mean?

Zamanında means “on time” or “in time.” Morphologically:
zaman (time) + -ın (3rd-person singular possessive) + -da (locative = “in/at”).
Literally “in its time,” used adverbially to convey punctuality.

What does yanıt veriyorum mean, and why two words instead of one?

Yanıt vermek is a compound verb:
yanıt (answer/response) + vermek (to give).
Turkish often uses such “noun + verb” combos rather than a single root. You could also say (“I am answering”), but is very common.