Learn Hardcore Malay: Saya asah pensel tumpul di ruang tamu. - I sharpen a blunt pencil in the living room.

sebatang pensel tumpul – a / one blunt pencil (using the classifier batang)
pensel tumpul itu – that blunt pencil / the blunt pencil
banyak pensel tumpul – many blunt pencils

How can I tell whether pensel tumpul here is singular or plural?

By itself, pensel tumpul is number-neutral; it could be one or more pencils. You only know from context or by adding markers:

sebatang pensel tumpul – one blunt pencil
dua batang pensel tumpul – two blunt pencils
banyak / beberapa pensel tumpul – many / several blunt pencils
semua pensel tumpul – all the blunt pencils

If there is no such word, you deliberately leave it vague, which is very common in Malay.

Why is it pensel tumpul and not tumpul pensel?

In Malay the normal order is:

Noun + Adjective

So you say:

pensel tumpul – blunt pencil
buku baharu – new book
rumah besar – big house

Putting the adjective first (tumpul pensel) is ungrammatical in standard Malay (except in a few fixed expressions that you can treat as exceptions). So pensel tumpul is the natural order.

Can I say pensel yang tumpul instead of pensel tumpul? Is there any difference?

Yes, you can say pensel yang tumpul, and it is grammatically correct. The difference is subtle:

pensel tumpul – simple description: a blunt pencil
pensel yang tumpul – slightly more focused/explicit: the pencil that is blunt

yang often:

makes the description more specific or contrastive, or
is used when the phrase is longer or more complex.

In this short, neutral sentence, most speakers would prefer the simpler pensel tumpul.

Why is the verb asah in its basic form? Could I say Saya mengasah pensel tumpul instead?

Both are possible:

Saya asah pensel tumpul.
Saya mengasah pensel tumpul.

asah is the root verb; mengasah is the same verb with the meN- prefix, which is very common in standard Malay.

Roughly:

mengasah sounds more standard/complete and is what you will often see in writing or careful speech.
Using the bare form asah like this is common in:
- casual speech,
- headlines, captions, or
- instruction-like sentences (e.g. Asah pensel di sini. – Sharpen pencils here.)

So Saya mengasah pensel tumpul di ruang tamu would be a very natural, fully standard version.

Does this sentence mean I sharpened, I am sharpening, or I will sharpen the pencil?

It could mean any of these. Malay verbs do not change form for tense. Saya asah pensel tumpul di ruang tamu is tenseless; the time is understood from context.

To be more explicit, Malay usually adds time words:

Tadi saya asah pensel tumpul di ruang tamu. – I sharpened … just now.
Sekarang saya sedang mengasah pensel tumpul di ruang tamu. – I am sharpening … now.
Nanti saya akan mengasah pensel tumpul di ruang tamu. – I will sharpen … later.

Without such markers, listeners infer the most likely time from the situation.

What exactly does di mean in di ruang tamu? Is it in, at, or on?

di is a general preposition for location and can translate as in, at, or on, depending on the noun:

di rumah – at home
di meja – on the table
di ruang tamu – in the living room

The English preposition you choose depends on the natural collocation in English, but di itself is not as specific as English prepositions.

What is the difference between di ruang tamu and di dalam ruang tamu?

Both are correct, and often interchangeable:

di ruang tamu – in/at the living room (normal, neutral)
di dalam ruang tamu – inside the living room

dalam literally means inside. di dalam ruang tamu can sound a bit more explicit about being inside that space, or slightly more formal, but in everyday speech di ruang tamu is what you will hear most.

Can I move di ruang tamu to the front or middle of the sentence?

Yes, Malay word order is quite flexible for location/time phrases. All of these are grammatical:

Saya asah pensel tumpul di ruang tamu. – neutral, very common.
Di ruang tamu, saya asah pensel tumpul. – emphasizes the location.
Saya di ruang tamu asah pensel tumpul. – possible, but feels less natural; you usually place di ruang tamu at the end or at the very beginning.

The default, most natural position is at the end.

What is the nuance of using Saya here instead of aku?

Both mean I, but they differ in formality:

Saya
- polite, neutral, and standard
- used with strangers, in formal situations, and also acceptable with friends
aku
- informal / intimate
- used with close friends, siblings, or in casual speech

In your example, Saya asah pensel tumpul di ruang tamu sounds polite-neutral and is safe in almost any context. Aku asah pensel tumpul di ruang tamu would sound more casual, like talking to a close friend.

Is asah always used with an object like pensel, or can it stand alone?

asah / mengasah is normally a transitive verb: you sharpen something. So you usually say:

Saya asah pensel. – I sharpen a pencil.
Dia mengasah pisau. – He/she sharpens a knife.

Using asah without an object (Saya asah.) sounds incomplete or unnatural unless the object is very obvious from the immediate context (for example, someone just asked, Kamu buat apa dengan pisau tu? – What are you doing with that knife? and you answer Asah. – Sharpening it).

Is the word pensel Malay-specific? I’ve also seen pensil.

Yes, spelling differs between Malay and Indonesian:

In Malay (Malaysia, Brunei, Singapore), the standard spelling is pensel.
In Indonesian, the standard spelling is pensil.

The meaning is the same: pencil. Since your sentence is Malay, pensel is the correct form.