Saya simpan masker di tas saya.

Both are acceptable, but the register differs:

• Saya menyimpan masker di tas saya. = standard/neutral, good for writing and formal speech.
• Saya simpan masker di tas saya. = common in conversation and headlines; the meN- prefix is dropped. It’s fine in everyday speech but less formal.

Indonesian doesn’t mark tense on verbs. Saya simpan… can mean past, present, or future depending on context. Add time/aspect words if needed:

• Past: Saya sudah/baru saja menyimpan masker di tas saya.
• Present progressive: Saya sedang menyimpan masker di tas saya. (formal) / Saya lagi nyimpen masker di tas saya. (informal)
• Future: Saya akan/nanti saya menyimpan masker di tas saya.

By default it’s number-neutral. To be explicit:

• A/one mask: satu masker (or sebuah masker, less common), often just masker if context is clear.
• The mask: masker itu or maskernya (definite/previously known).
• Masks: masker (context), or mark it with beberapa masker, banyak masker, or masker-masker (reduplication, more formal/literary).

Here di is a preposition meaning “at/in/on” and is written separately: di tas. The passive prefix di- attaches to verbs and is written together, e.g., disimpan “is/was kept.” Compare:

• Preposition: di tas saya = in my bag.
• Passive: Masker itu disimpan di tas saya.

You can vary possession:

• Neutral/formal: tas saya.
• Colloquial enclitics: tasku (my bag), tasmu (your bag), tasnya (his/her/that bag).
• Super informal/Jakarta: tas gue, tas kamu/lu.
  You don’t have to repeat the possessor if context is clear; di tas might be understood as “in my bag” if already established.

Yes, for emphasis/topic:

• Neutral: Saya menyimpan/simpan masker di tas saya.
• Object fronting (emphasis on the mask): Masker itu saya simpan di tas saya.
• Passive: Masker itu disimpan di tas saya (oleh saya).
  Avoid putting the location before the object right after the verb (e.g., “Saya simpan di tas saya masker”)—that sounds odd.

With placement verbs, Indonesian usually uses di for the end location:

• Natural: Saya menaruh/menyimpan masker di tas saya. If you want to emphasize motion into, use ke dalam:
• Saya menaruh masker ke dalam tas saya.
  But “menyimpan masker ke tas saya” is not idiomatic; prefer di tas or ke dalam tas depending on nuance.

Attach -nya to the verb or repeat the noun with -nya:

• Saya menyimpannya di tas saya. = I kept/put it in my bag.
• Maskernya saya simpan di tas saya. = I kept the mask in my bag.

Use aspect/tense markers:

• Past/completed: Saya sudah menyimpan masker di tas saya.
• Just now: Saya baru saja menyimpan masker di tas saya.
• Ongoing: Saya sedang menyimpan masker di tas saya. / Saya lagi nyimpen masker di tas saya.
• Future: Saya akan menyimpan masker di tas saya. / Nanti saya menyimpan/menaruh masker di tas saya.

Yes. The active prefix meN- assimilates based on the first consonant of the root:

• Root: simpan
• meN- + s → meny-
  • (root) = menyimpan
    This is why it’s not “mensimpan.” In casual speech you’ll also hear nyimpen (colloquial reduction).

Use the base verb, optionally with please:

• (Tolong) simpan masker di tas saya.
• More neutral everyday: (Tolong) taruh masker di tas saya.

Not in the simple sentence. yang introduces a relative clause:

• Masker yang saya simpan di tas saya = “the mask that I keep in my bag.”
  For the standalone sentence, you don’t need yang.