nǐ gǎnmào le, yào duō hē shuǐ.

In 你感冒了, 了 does not simply mean past tense.

Here, 了 marks a new situation or change of state:

• before: you were fine
• now: you have a cold

So 你感冒了 means something like:

• You’ve caught a cold
• You have a cold now

Without 了, 你感冒 sounds less natural as a normal statement about your current condition.

It can behave like both, depending on the sentence.

• As a noun, 感冒 means a cold
• As a verb/predicate, it can mean to catch a cold or to have a cold

In 你感冒了, it works as the main predicate of the sentence, so it is functioning more like to have/catch a cold.

Because 感冒 is not a noun being linked by 是 in this sentence.

Mandarin often states conditions directly with the predicate:

• 你感冒了 = You have a cold
• 我累了 = I’m tired
• 他病了 = He got sick

Using 是 here would usually sound unnatural unless you are emphasizing or contrasting something in a special context.

Here, 要 means should, need to, or ought to.

So:

• 要多喝水 = should drink more water

It does not mean literal desire here.

Compare:

• 我要喝水 = I want to drink water
• 你感冒了，要多喝水 = You have a cold, so you should drink more water

Context tells you which meaning 要 has.

多 means more here.

So 多喝水 literally means drink more water.

It suggests increasing the amount or frequency of drinking water. In natural English, it could be:

• Drink more water
• Drink plenty of water

Usually it means drink more water, especially as advice.

But in natural translation, it can also come across as drink plenty of water.

A helpful comparison:

• 多喝水 = drink more water / drink extra water
• 喝很多水 = drink a lot of water

The first one is especially common in advice.

Yes. The subject is still you.

Chinese often omits repeated subjects when they are obvious from context.

Full version:

• 你感冒了，你要多喝水。

Natural version:

• 你感冒了，要多喝水。

This kind of omission is very common in Mandarin.

Yes.

Depending on context, 你感冒了 can be understood as:

• You caught a cold
• You’ve got a cold
• You have a cold now

Because of 了, there is often a sense that this is a newly relevant condition.

A native speaker would say:

Nǐ gǎnmào le, yào duō hē shuǐ.

A useful pronunciation note:
你 is third tone, and 感 is also third tone. When two third tones come together, the first one usually changes in actual speech.

So 你感 is often pronounced more like:

ní gǎn...

This is a normal tone sandhi pattern.

Yes.

Similar sentences include:

• 你感冒了，应该多喝水。
  You have a cold; you should drink more water.

• 你感冒了，得多喝水。
  You have a cold; you need to drink more water.

• 你感冒了，最好多喝水。
  You have a cold; you’d better drink more water.

In your original sentence, 要多喝水 is very natural and common.

Mandarin usually does not use articles like a, an, or the the way English does.

So 水 simply means water, and context tells you how specific it is.

• 喝水 = drink water
• 多喝水 = drink more water

You do not need an article here.

It is best understood as advice.

• 你感冒了 = statement of condition
• 要多喝水 = recommendation / advice

So the whole sentence feels like:

You have a cold, so you should drink more water.

It is not as harsh as a direct order.