Berat badannya rendah selepas dehidrasi.

Malay often leaves out the verb to be in sentences like this.

So:

Berat badannya rendah
literally looks like His/her body weight low

but naturally means:

His/her body weight is low

This is completely normal in Malay. Adjectives like rendah can directly act as the predicate without needing a separate word for is.

Badannya is made up of:

• badan = body
• -nya = his / her / their / the person in question

So badannya means his body, her body, or more generally the person's body, depending on context.

In this sentence, it appears inside the expression berat badannya, which means his/her body weight.

Both are possible, but they sound a little different.

• berat badannya = his/her body weight
• berat badan dia = his/her body weight

Using -nya is often a bit more compact and slightly more neutral or written in tone.
Using dia can sound more conversational.

So the sentence with badannya is very normal, especially in neutral or semi-formal Malay.

Yes. Berat badan is the standard expression for body weight.

Even though the individual words are:

• berat = heavy / weight
• badan = body

together berat badan functions as a set phrase meaning body weight.

So berat badannya is best understood as his/her body weight, not by translating each word too separately.

Yes, rendah literally means low, and that works well here because body weight can be treated as a measured value.

So berat badannya rendah means that the person's body weight is low.

This is different from talking about how a person looks. The sentence is about the level/value of body weight, not directly about appearance.

Because kurus and rendah describe different things.

• kurus = thin / skinny
• rendah = low

Kurus describes a person's body shape or appearance.
Rendah describes a low level or value.

So:

• Dia kurus = He/She is thin
• Berat badannya rendah = His/Her body weight is low

They are related ideas, but not the same thing.

Usually no, not in this kind of sentence.

Ringan means light in the sense of not heavy, but it is not the usual word for a low measured value like body weight in a medical or factual statement.

For body weight as a measurement, rendah is more suitable than ringan.

So berat badannya rendah sounds much better than berat badannya ringan.

It depends on what you want to emphasize.

• rendah = low
• menurun = decreased / went down

So:

• Berat badannya rendah selepas dehidrasi focuses on the person's weight being low after dehydration.
• Berat badannya menurun selepas dehidrasi focuses on the weight dropping after dehydration.

If the intended idea is that dehydration caused the weight to go down, menurun may sound more natural.
If the idea is simply that the weight was low afterward, rendah works.

Strictly speaking, selepas means after, so it mainly gives a time relationship.

So selepas dehidrasi means after dehydration.

It may suggest a connection, but by itself it does not explicitly say because of dehydration. If you want to show cause more clearly, Malay could use:

• akibat dehidrasi = as a result of dehydration
• kerana dehidrasi = because of dehydration

Also, if you want to make the event more explicit, you could say:

• selepas mengalami dehidrasi = after experiencing dehydration

Malay does not mark tense the way English does.

The sentence itself does not contain a special past-tense form. Instead, time is understood from context and from words like selepas.

So this sentence can be understood as something like:

• his/her weight was low after dehydration
• his/her weight is low after dehydration

The exact English tense depends on the wider context.

No. -nya does not show gender.

It can mean:

• his
• her
• their
• sometimes just the person's or that person's

Malay generally does not mark grammatical gender the way English does, so you need context to know whether the person is male or female.

Yes. Malay is fairly flexible with time expressions.

The original sentence:

Berat badannya rendah selepas dehidrasi.

can also be rearranged as:

Selepas dehidrasi, berat badannya rendah.

Both are natural. The second version puts more emphasis on the time frame after dehydration.