Saya letak kotak di bawah meja.

Malay verbs do not change form for tense. Instead, context or time words (like sudah for past or akan for future) indicate when an action happens.

• “Saya letak kotak…” by itself is often understood as past if you’ve been talking about what you just did.
• To make it explicit past, you could say Saya sudah letak kotak di bawah meja.
• For future: Saya akan letak kotak di bawah meja.

“Saya letak…” is colloquial Malay. In informal speech, people often drop the me- prefix and the -kan suffix. The full formal verb is meletakkan, so formally you could say:

• Saya meletakkan kotak di bawah meja.
  Both mean “I put the box under the table,” but the version with me-…-kan sounds more formal or written.

Here, di is part of the compound preposition di bawah, meaning “under.” It is not the passive-voice marker (which also uses di- on verbs). In this phrase:

• di
  • bawah = “under”
• meja = “table”
  So di bawah meja = “under the table.”

Malay does not have articles like a or the. Nouns stand alone, and you rely on context to know if you mean “a box,” “the box,” or “that box.” If needed, you can add demonstratives or classifiers:

• sebuah kotak = “a box”
• kotak itu = “that box”

Typical Malay word order is: Subject – Verb – Object – (Place) – (Time).
In Saya letak kotak di bawah meja:

• Subject = Saya (“I”)
• Verb = letak (“put”)
• Object = kotak (“box”)
• Place = di bawah meja (“under the table”)
  There’s no time phrase here; if you wanted to add one, it would come at the end.

Yes. Fronting kotak makes it a topic-comment structure and shifts the emphasis to the box. It’s still grammatical:

• Kotak, saya letak di bawah meja.
  But the neutral order is Saya letak kotak….

Dropping di (saying bawah meja) is very informal or dialectal. In standard Malay, you should keep di for prepositional phrases:

• Correct: di bawah meja
• Very colloquial: bawah meja