La cisterna pierde agua otra vez, así que llamaré a la fontanera.

In this sentence, la cisterna most naturally means the toilet cistern / toilet tank in Spain.

More generally, cisterna can also mean a tank or reservoir, but in everyday home-repair contexts in Spain, it often refers to the part of the toilet that holds water.

So here the idea is:

• La cisterna pierde agua = The toilet cistern is leaking

Literally, pierde agua means loses water, but in Spanish this is a very common way to say that something leaks.

Examples:

• La cisterna pierde agua. = The cistern leaks / is leaking.
• El grifo pierde agua. = The tap is leaking.
• La tubería pierde agua. = The pipe is leaking.

So Spanish often expresses this idea with perder + noun:

• perder agua = to leak water
• perder aceite = to leak oil

Both are possible, but pierde is the more natural default here.

Spanish often uses the simple present for things happening now, especially when describing a problem or situation:

• La cisterna pierde agua otra vez.

This can mean:

• it leaks
• it’s leaking
• it keeps leaking

If you say está perdiendo agua, that puts more emphasis on the action being in progress right now at this moment.

So:

• pierde agua = normal, general, idiomatic
• está perdiendo agua = more explicitly “it is leaking right now”

Otra vez means again.

Yes, de nuevo also works:

• La cisterna pierde agua otra vez.
• La cisterna pierde agua de nuevo.

Both mean essentially the same thing here.
In everyday speech, otra vez is extremely common.

A small nuance:

• otra vez can sometimes feel a bit more conversational or emotional, especially if you are annoyed: Again?!
• de nuevo can sound slightly more neutral in some contexts

But in this sentence, either one is fine.

Así que means so, therefore, or which is why.

It connects the problem to the consequence:

• La cisterna pierde agua otra vez, así que llamaré a la fontanera.
• The cistern is leaking again, so I’ll call the plumber.

It is a very common way to express a result or decision.

Compare:

• porque = because
• así que = so

For example:

• Llamaré a la fontanera porque la cisterna pierde agua.
  = I’ll call the plumber because the cistern is leaking.
• La cisterna pierde agua, así que llamaré a la fontanera.
  = The cistern is leaking, so I’ll call the plumber.

Llamaré is the simple future: I will call.

Spanish also often uses ir a + infinitive for future plans:

• Voy a llamar a la fontanera. = I’m going to call the plumber.

Both are correct here.

A rough difference:

• llamaré = a bit more concise; can sound like a decision or statement of intention
• voy a llamar = very common in everyday speech; often feels more immediate or conversational

In modern spoken Spanish, voy a llamar is extremely frequent, but llamaré is still perfectly normal.

That a is the personal a.

In Spanish, when the direct object is a specific person (or sometimes a pet or personified being), you usually put a before it:

• Llamaré a la fontanera.
• Vi a María.
• Conozco a tu hermano.

Even though English does not use anything like this, Spanish normally does.

So:

• llamar a alguien = to call someone

You would not normally omit it here.

Because fontanera is the feminine form of fontanero.

• el fontanero = the male plumber
• la fontanera = the female plumber

Spanish job nouns usually agree with the person’s gender. So if the plumber is a woman, fontanera is correct.

If the gender were unknown or if you were referring to a man, you would usually say:

• Llamaré al fontanero.

In Spain, fontanero / fontanera is the standard word for plumber.

Yes, it is especially associated with Spain.

In Spain:

• fontanero / fontanera = plumber

In many parts of Latin America, other words are more common, such as:

• plomero
• sometimes regional alternatives depending on the country

So if you are learning Spanish from Spain, fontanero/fontanera is exactly the word you want.

Spanish often uses the definite article (el, la, los, las) when talking about a specific thing that is already understood from the context.

Here, la cisterna means:

• the cistern
  that is, the one in the house / bathroom we are talking about.

You could also say:

• Mi cisterna pierde agua otra vez.

But that is less natural in many everyday situations unless you want to emphasize that it is your cistern.

And saying just cisterna pierde agua would be ungrammatical here; Spanish normally needs an article or another determiner.

Because Spanish often drops subject pronouns when they are already clear from the verb ending.

• llamaré already means I will call
• so yo is not necessary

You could say:

• ...así que yo llamaré a la fontanera.

But that would usually add emphasis or contrast, as if you were saying:

• ...so I’ll call the plumber (not someone else)

In neutral sentences, Spanish usually leaves out the subject pronoun.

Yes, but they are not exactly the same in tone or structure.

Common options include:

• La cisterna pierde agua. = The cistern leaks / is leaking.
• La cisterna tiene una fuga. = The cistern has a leak.
• Hay una fuga en la cisterna. = There is a leak in the cistern.

These are all natural.

Se sale agua can exist, but it sounds less direct and is not the most natural first choice here.
For a household problem, pierde agua is one of the most idiomatic ways to say it.

The difference is mainly one of style and immediacy.

• Llamaré a la fontanera. = I’ll call the plumber.
• Voy a llamar a la fontanera ahora mismo. = I’m going to call the plumber right now.

The second version sounds more immediate and explicit because of:

• voy a llamar = going to call
• ahora mismo = right now

The original sentence is more neutral and compact.

Yes, they are important.

• así has a written accent on í
• llamaré has a written accent on é

These accents show the correct stress pattern and are part of the standard spelling.

For example:

• llamaré = I will call
• llamare without the accent would be incorrect

So when writing Spanish, you should always include these accents.

The original sentence is already natural. But in casual spoken Spanish from Spain, you might also hear:

• La cisterna pierde agua otra vez, así que voy a llamar a la fontanera.
• La cisterna vuelve a perder agua, así que llamaré a la fontanera.
• La cisterna está perdiendo agua otra vez, así que voy a llamar a la fontanera.

A particularly natural variant is:

• La cisterna vuelve a perder agua...

That literally means the cistern is leaking again or has started leaking again. It is very idiomatic for repeated problems.