Aku nggak sempat membalas pesan tadi pagi.

Both mean I, but they differ in tone:

• aku = informal, personal, common with friends, family, partners, casual texting.
• saya = more neutral/polite, used with strangers, in formal settings, at work (depending on workplace culture), customer service, etc.

You can swap them in this sentence:

• Saya nggak sempat membalas pesan tadi pagi. (still informal because of nggak)
• For a consistently more formal version: Saya tidak sempat membalas pesan tadi pagi.

nggak (also written gak, enggak) is the very common colloquial negator meaning not / don’t / didn’t.
In more formal or careful Indonesian, you’d typically use tidak:

• Aku tidak sempat membalas pesan tadi pagi.

In everyday speech and chats, nggak is extremely normal.

sempat expresses having (enough) time / having the chance / managing to do something. With negation (nggak/tidak), nggak sempat + verb means:

• you didn’t have time to do it
• you didn’t get a chance to do it (often because you were busy)

Pattern:

• (Subj) (nggak/tidak) sempat + Verb (+ Object)

Example:

• Aku sempat makan. = I managed to eat / I had time to eat.
• Aku nggak sempat makan. = I didn’t have time to eat.

balas is the root meaning reply/return/respond.
membalas is the active verb form with the meN- prefix, common in standard Indonesian when you have an object right after it.

Both can be used in casual speech:

• Aku nggak sempat membalas pesan tadi pagi. (more “standard”)
• Aku nggak sempat balas pesan tadi pagi. (more casual)

The prefix is meN-, which marks an active verb. Its shape changes depending on the first sound of the root.

Root: balas (starts with b)
Rule: meN- + b… → mem- + b…
So you get: mem + balas → membalas

This is a common pattern:

• baca → membaca
• buat → membuat
• beli → membeli

Good catch: pesan has multiple common uses depending on context: 1) pesan = a message (text/DM/email content)
2) pesan = an order (e.g., food order) / the act of ordering (memesan)
3) pesan = advice/moral/message (as in “a lesson”)

In membalas pesan, the context strongly points to message (replying to a message).

Indonesian generally doesn’t change verb forms for past tense. Time is usually shown by:

• time expressions (tadi pagi, kemarin, minggu lalu)
• context
• optional aspect words like sudah (already), baru (just), belum (not yet)

Here, tadi pagi anchors the action in the past without changing the verb:

• Aku nggak sempat membalas pesan tadi pagi.

They’re very close and often interchangeable:

• tadi pagi = “this morning (earlier)” with tadi introducing the “earlier today” idea
• pagi tadi = “earlier this morning” with focus starting on pagi

Both sound natural:

• Aku nggak sempat membalas pesan tadi pagi.
• Aku nggak sempat membalas pesan pagi tadi.

Yes, in context Indonesians often omit the subject, especially in conversation/texting:

• Nggak sempat membalas pesan tadi pagi. = (I) didn’t have time to reply this morning.

Whether it’s clear depends on the conversation. If it might be ambiguous, keep aku/saya.

They’re different in nuance:

• nggak sempat = didn’t have time (often implies the chance is gone, or simply stating fact)
• belum sempat = haven’t had time yet (often implies you still might do it later)

Compare:

• Aku nggak sempat membalas pesan tadi pagi. = I didn’t have time to reply this morning.
• Aku belum sempat membalas pesan tadi pagi. = I haven’t gotten around to replying to the message from this morning (and may still reply now).

Indonesian often uses a direct object where English uses a preposition:

• membalas pesan = literally “reply the message” → natural Indonesian for “reply to the message”

You can also make the recipient explicit with ke:

• membalas pesan ke kamu = reply to your message / reply to you (via message)

But the sentence as given is complete without it.

More formal:

• Saya tidak sempat membalas pesan tadi pagi.
• Even more formal: Saya tidak sempat membalas pesan Anda tadi pagi. (to a customer/older person)

More casual/texty:

• Aku gak sempat bales pesan tadi pagi.
• Tadi pagi aku gak sempet bales chat. (chat is also commonly used)