Si regues aquest test cada dia, la planta començarà a créixer aviat.

Because regues is the 2nd person singular present indicative of regar: you water.

In Catalan, after si in a real, possible condition, you normally use the present tense:

• Si regues aquest test cada dia... = If you water this pot every day...

Then the main clause can use the future:

• ...la planta començarà a créixer aviat. = ...the plant will start to grow soon.

So the tense pattern is very similar to English: If you water..., it will grow...

Normally, no.

In standard Catalan, after si meaning if in this kind of real condition, you do not usually use the future tense. You use the present:

• Si regues aquest test cada dia, la planta començarà a créixer aviat.

Not:

• Si regaràs...

So a good rule is:

• si + present
• main clause often future

Here test is a false friend.

In Catalan, un test means a flowerpot / plant pot.

So:

• aquest test = this pot

It does not mean an exam or quiz here.

Also, Catalan can naturally say regar un test, even though in English we often say water the plant. The idea is that you water the pot and therefore the plant in it.

Because Catalan words have grammatical gender, and the determiner has to agree with the noun.

• test is masculine singular → aquest test
• planta is feminine singular → la planta

Compare:

• aquest test = this pot
• aquesta planta = this plant

So aquest and aquesta are the masculine and feminine forms of this.

Because Catalan often drops subject pronouns when they are not needed.

The verb ending already shows the subject:

• regues = you water

So tu is unnecessary unless you want emphasis or contrast.

For example:

• Si regues aquest test cada dia... = neutral
• Si tu regues aquest test cada dia... = more emphasis on you

This is very normal in Catalan.

Because in Catalan, començar is normally followed by a + infinitive when it means to start doing something.

So:

• començar a créixer = to start growing
• començar a parlar = to start speaking
• començar a ploure = to start raining

That a is just part of the usual pattern after començar.

Començarà means will start.

It is the 3rd person singular future of començar:

• la planta començarà = the plant will start

The ç is there to keep the s sound before a.

In Catalan:

• c before a, o, u would normally sound like k
• ç gives the soft s sound

So:

• començarà is pronounced with an s sound, not a k sound

The final accent à shows the stress falls on the last syllable: co-men-ça-RÀ.

In créixer, the x is pronounced like sh.

So for an English speaker, a rough approximation is something like:

• CRAY-sher or CRAY-shuh, depending on the accent

A few useful points:

• créixer = to grow
• the x here is not like English x in box
• it sounds more like sh

So créixer does not sound like krek-ser.

Yes.

You can also say:

• La planta començarà a créixer aviat si regues aquest test cada dia.

The meaning stays the same.

A useful punctuation note:

• when the si clause comes first, a comma is commonly used
• when it comes second, the comma is often omitted

So both are fine:

• Si regues aquest test cada dia, la planta començarà a créixer aviat.
• La planta començarà a créixer aviat si regues aquest test cada dia.

Then you would usually use a different pattern:

• Si reguessis aquest test cada dia, la planta començaria a créixer aviat.

This is more hypothetical or less direct.

Compare:

• Si regues..., la planta començarà...
  real/possible condition: If you water..., the plant will start...

• Si reguessis..., la planta començaria...
  hypothetical condition: If you watered..., the plant would start...

So the original sentence is the more straightforward, real possibility.

Here it means soon.

• la planta començarà a créixer aviat = the plant will start to grow soon

Depending on context, aviat can sometimes be translated as early, but in this sentence soon is the natural meaning.

So if you see aviat with a future result like this, soon is usually the best choice.