Learn Hardcore Indonesian: Hujan deras bisa merusak buku di tas. - Heavy rain can damage the books in the bag.

bisa and dapat
In this kind of sentence, bisa and dapat are usually interchangeable, especially in casual speech:
- Hujan deras bisa merusak buku di tas.
- Hujan deras dapat merusak buku di tas.
  Both = Heavy rain can damage the books in the bag.
Some speakers feel dapat is a bit more formal or written, but the meaning is basically the same here.
boleh
Boleh is about permission (may), not ability:
- Buku boleh di tas. = The book may be in the bag / It’s allowed.
  So boleh would be wrong in this sentence, because rain doesn’t have “permission”; we are talking about ability/possibility, so bisa or dapat is correct.

Why is it hujan deras and not deras hujan?

In Indonesian, the normal word order is:

Noun + describing word (adjective)

So:

hujan deras = heavy rain
(hujan = rain, deras = heavy, pouring, intense)

Putting the adjective first (deras hujan) would sound wrong or poetic/marked. For a normal sentence, you say hujan deras, just like buku baru (new book), rumah besar (big house), tas merah (red bag).

What is the difference between hujan deras and hujan lebat?

Both describe heavy rain, and they overlap a lot.

hujan deras
- Very common in everyday speech.
- Focus on intensity/force of the rain.
hujan lebat
- Also common, slightly more “descriptive” or “written” for some speakers.
- Also means heavy rain.

You can usually swap them without changing the meaning much:

Hujan deras bisa merusak buku di tas.
Hujan lebat bisa merusak buku di tas.

Both are natural and mean essentially the same thing.

Why is it merusak and not just rusak?

Rusak and merusak are related but have different functions:

rusak (adjective / intransitive verb) = broken, damaged
- Bukunya rusak. = The book is damaged / broken.
  Here rusak describes the book’s condition.
merusak (transitive verb) = to damage, to break something
- Hujan deras bisa merusak buku. = Heavy rain can damage the books.
  Here merusak takes an object: buku.

So in your sentence, something (the rain) is causing damage to something else (the books), so the transitive form merusak is needed, not just rusak.

Does bisa merusak mean “is able to damage” or “might damage / could damage”? How strong is it?

Bisa covers both ideas:

ability / capability:
- Heavy rain is capable of damaging the books.
possibility / likelihood:
- Heavy rain might / can (in the sense of “often does”) damage the books.

In everyday Indonesian, bisa doesn’t clearly separate these two meanings; the context does. Here it suggests a real, practical possibility: if it rains heavily, your books are likely to get damaged if they are in the bag and not protected.

How can I say this in the past or future? Indonesian doesn’t have tenses, right?

Correct: Indonesian doesn’t change the verb form for tense. You add time words instead.

Past (Heavy rain damaged the books in the bag):
- Tadi hujan deras merusak buku di tas.
  (Earlier, heavy rain damaged the books in the bag.)
- Kemarin hujan deras merusak buku di tas.
  (Yesterday heavy rain damaged the books in the bag.)
Future (Heavy rain will damage the books in the bag):
- Nanti hujan deras bisa merusak buku di tas.
- Besok hujan deras bisa merusak buku di tas.

Note:

You can keep bisa if you mean “it can / it might happen”.
If you are stating a fact that already happened, you usually drop bisa:
- Tadi hujan deras merusak buku di tas.

Does buku here mean “book” or “books”? How do you show plural in Indonesian?

By itself, buku is number-neutral. It can mean book or books, depending on context.

To be more specific:

Singular:
- sebuah buku = a book (one book)
- satu buku = one book
Plural:
- buku-buku = books (general plural)
- beberapa buku = several books
- banyak buku = many books

So your sentence could be understood as:

Heavy rain can damage the book in the bag.
or
Heavy rain can damage the books in the bag.

If you want to make it clearly plural:

Hujan deras bisa merusak buku-buku di tas.
Hujan deras bisa merusak banyak buku di tas.

What is the difference between di tas and di dalam tas?

Both are possible, but there is a nuance:

di tas
Literally “at the bag / on the bag / in the bag”.
In everyday speech, di tas often already implies “in the bag”, especially when talking about buku, HP, dompet, etc.
di dalam tas
Literally “inside the bag”.
This is more explicit and emphasizes that the books are inside, not just on or near the bag.

So:

Hujan deras bisa merusak buku di tas.
= Heavy rain can damage the books in the bag. (normal, everyday)
Hujan deras bisa merusak buku di dalam tas.
= Heavy rain can damage the books inside the bag. (extra clear about “inside”)

Is di always necessary before tas? Can I say buku dalam tas?

There are two slightly different patterns:

di + location noun (preposition + noun)
- di tas = at/in the bag
- di rumah = at home
- di meja = on the table
dalam + noun can behave like a preposition (“in”) or a preposition-like adjective (“inside”):
- buku di dalam tas (full form, very clear)
- In more fluent speech/writing, buku dalam tas is also seen and usually understood as “books in the bag”, but buku di dalam tas is safer and very standard.

In your level as a learner, it’s good practice to always use di when you mean “at/in/on”:

buku di tas
buku di dalam tas

Where do possessive words go? How do I say “the books in my bag” here?

You have a few natural options:

buku di tas saya
- Hujan deras bisa merusak buku di tas saya.
  = Heavy rain can damage the books in my bag.
  (saya follows the noun it possesses.)
buku di dalam tas saya
- Same meaning, more explicit “inside”.
Using the suffix -ku for “my” (quite common, a bit more informal/intimate):
- Hujan deras bisa merusak bukuku di tas.
  = Heavy rain can damage my book in the bag.
- Hujan deras bisa merusak buku di tasku.
  = Heavy rain can damage the books in my bag.

All are correct. The most neutral, learner-friendly version is:

Hujan deras bisa merusak buku di tas saya.

Can I move di tas to another position, like Buku di tas bisa dirusak hujan deras?

You can move di tas, but you also need to watch the rest of the structure.

Your original sentence:

Hujan deras bisa merusak buku di tas.
(Subject) (modal) (verb) (object + location)

Other natural options:

Buku di tas bisa rusak karena hujan deras.
= The books in the bag can get damaged because of heavy rain.
(Subject + location) (modal) (become) (reason)

Here we changed the structure slightly:

bisa rusak instead of bisa merusak
karena hujan deras (because of heavy rain)

Simply saying:

Buku di tas bisa dirusak hujan deras.

is grammatically understandable, but:

It uses the passive dirusak.
It sounds more formal/bookish and slightly unnatural without oleh:
- More natural passive: Buku di tas bisa dirusak oleh hujan deras.

For everyday speech, your original active sentence is clearer and more typical:

Hujan deras bisa merusak buku di tas.

Is this sentence formal, informal, or neutral? Can I use it in writing?

The sentence:

Hujan deras bisa merusak buku di tas.

is neutral and perfectly acceptable in both speech and writing.

It’s not slangy or very informal.
It’s not very formal or bureaucratic either.
You can use it in:
- Conversation
- A textbook or article
- A notice or warning (maybe slightly adjusted)

In a very formal written context, someone might prefer dapat over bisa, e.g.:

Hujan deras dapat merusak buku di tas.

But both are fine and easily understood.