Learn Hardcore Malay: Saya bayar bil di kaunter. - I pay the bill at the counter.

saya: polite/neutral “I,” safe in most situations.
aku: intimate/informal, used with close friends/family; avoid with strangers.
awak: “you,” casual; fine with peers; can sound too familiar with elders/strangers.
Anda: formal/polite “you,” common in customer-facing language and writing; less in everyday face-to-face conversation. Regional/other forms exist (e.g., kamu , gua ), but the above are the core ones to master first.

Both are correct. bayar is very common in everyday speech and neutral in tone. membayar (with the meN- prefix) sounds more formal and is frequent in writing, signage, and official contexts.

Casual: Saya sudah bayar bil di kaunter.
Formal: Saya telah membayar bil di kaunter. After auxiliaries, both can work, with formality shifting accordingly:
nak/mahu/ingin/hendak bayar vs hendak membayar
boleh bayar vs boleh membayar (the first is more natural in speech)

How do I show past, present, or future time? Malay verbs don’t change form.

Use time/aspect markers or time words:

Past/already: sudah or telah (formal) — Saya sudah/telah bayar bil di kaunter.
In progress: sedang — Saya sedang bayar bil di kaunter.
Future: akan — Saya akan bayar bil di kaunter.
Time words: tadi/semalam (earlier/yesterday), nanti/esok (later/tomorrow) — e.g., Saya bayar bil di kaunter tadi.

Can I drop the subject Saya?

Yes, in casual speech when context is clear: Bayar bil di kaunter. However, for learners and in formal writing, keep Saya to avoid ambiguity.