El agua debe drenar bien.

Because agua is feminine, but in the singular it takes el instead of la when the stressed sound at the start is a- or ha-.

So you say:

• el agua
• el águila
• el hambre

This is done for sound reasons, to avoid la agua, which is awkward to pronounce.

But the noun is still feminine, so other words agree as feminine:

• el agua fría
• mucha agua
• esta agua

And in the plural, it goes back to las:

• las aguas

Debe is the third person singular form of deber in the present tense.

The subject is el agua, which is singular, so the verb must also be singular:

• el agua debe
• not el agua deben

Here is the pattern:

• yo debo
• tú debes
• él/ella/usted debe
• nosotros debemos
• vosotros debéis
• ellos/ellas/ustedes deben

So debe is used because el agua is treated like it in English.

It can mean either, depending on context.

In practical instructions like this, debe + infinitive often means something between should and must:

• El agua debe drenar bien = The water should drain well / The water must drain well

The exact force depends on the situation:

• If it is advice, it may feel like should
• If it is a requirement, it may feel like must

If you want a clearly softer idea, Spanish often uses debería:

• El agua debería drenar bien = The water should drain well

Because after debe, Spanish uses the infinitive.

So the structure is:

• debe + infinitive
• debe drenar

This is like English:

• should drain
• not should drains

Compare:

• El agua drena bien = The water drains well
• El agua debe drenar bien = The water should/must drain well

So drena would only be correct if it were the main verb by itself, without debe.

Bien is an adverb, and it modifies the verb drenar.

It means well:

• drenar bien = to drain well

This is the same idea as:

• hablar bien = to speak well
• funcionar bien = to work well

A common learner mistake is to use bueno here, but bueno is an adjective, not an adverb.

So:

• drena bien ✅
• drena bueno ❌

Because deber + infinitive normally expresses obligation, necessity, or recommendation:

• debe drenar bien

Traditionally, deber de + infinitive is used for probability or assumption:

• Debe de estar en casa = He/She is probably at home

So in careful standard usage:

• debe drenar bien = obligation/recommendation
• debe de... = probability

In real-life Spanish, people do sometimes mix these patterns, but for learners it is best to keep this distinction.

Normally, no.

Spanish usually needs the article here:

• El agua debe drenar bien

English often allows a bare noun:

• Water should drain well

But Spanish usually prefers the article with a generic or general subject like this.

So even though English says just water, Spanish says el agua.

Here it is being used intransitively.

That means the water itself is the thing doing the draining:

• El agua debe drenar bien = The water should drain well

But drenar can also be transitive, where something drains something else:

• El tubo drena el agua = The pipe drains the water

So the verb can work both ways, depending on the sentence.

Yes. Tiene que drenar bien is also possible and often sounds very natural.

Compare:

• debe drenar bien
• tiene que drenar bien

Both can mean should/must drain well, but there is a slight difference in feel:

• debe can sound a bit more formal, technical, or written
• tiene que can sound a bit more direct and conversational

In many situations, either works.

You can tell from the words around it.

Even though it takes el in the singular, agreement stays feminine:

• el agua fría
• esta agua
• mucha agua

If it were masculine, you would expect:

• el agua frío ❌
• este agua ❌

So the article changes for pronunciation, but the gender does not change.