Se non sai nuotare, il bagnino ti aiuta.

Se means if. It introduces a condition:

• Se non sai nuotare = If you don’t know how to swim

This is a very common pattern in Italian:

• Se studi, impari. = If you study, you learn.
• Se piove, resto a casa. = If it rains, I stay home.

So in your sentence, the first part gives the condition, and the second part gives the result.

In Italian, non usually goes directly before the verb to make it negative.

• sai = you know
• non sai = you do not know

So:

• non sai nuotare = you do not know how to swim

This is the normal position for non in simple sentences.

Sai is the second person singular form of sapere:

• sapere = to know
• io so = I know
• tu sai = you know

Here, sapere + infinitive means to know how to do something.

So:

• sai nuotare = you know how to swim

This is an important pattern in Italian:

• So cucinare. = I know how to cook.
• Sai guidare? = Do you know how to drive?

Because sapere + infinitive means to know how to, while potere means to be able to / can.

Compare:

• Non sai nuotare = You don’t know how to swim
• Non puoi nuotare = You cannot swim / you are not allowed or not able to swim

So non sai nuotare focuses on lack of skill or knowledge, not permission.

After sapere when it means to know how to do something, Italian uses the infinitive.

• sapere nuotare = to know how to swim
• sapere parlare italiano = to know how to speak Italian
• sapere usare il computer = to know how to use the computer

So nuotare stays in its basic dictionary form because it depends on sai.

Italian often leaves out subject pronouns because the verb ending already shows who the subject is.

• sai already means you know

So (tu) non sai nuotare and non sai nuotare both mean the same thing.
The pronoun tu can be added for emphasis, but it is not necessary.

Il bagnino means the lifeguard.

Breakdown:

• il = the
• bagnino = lifeguard

It is a masculine singular noun, so it takes il.

The feminine form is usually:

• la bagnina = the female lifeguard

Ti is the unstressed object pronoun meaning you.

• il bagnino aiuta te = the lifeguard helps you
• il bagnino ti aiuta = the lifeguard helps you

In normal Italian, the short pronoun usually comes before the conjugated verb:

• mi aiuta = helps me
• ti aiuta = helps you
• ci aiuta = helps us

So ti is placed before aiuta because that is the standard word order for object pronouns.

Here it is a direct object.

In Italian, aiutare normally takes a direct object:

• Aiuto Marco. = I help Marco.
• Ti aiuto. = I help you.

That may feel a little different from English learners’ expectations, because English sometimes makes people think of help as needing to, but in Italian it does not:

• not aiutare a qualcuno in this basic sense
• but aiutare qualcuno

So ti is the direct object pronoun.

Aiuta is the third person singular present tense of aiutare.

Conjugation:

• io aiuto = I help
• tu aiuti = you help
• lui/lei aiuta = he/she helps

Since the subject is il bagnino (the lifeguard), Italian uses aiuta:

• il bagnino ti aiuta = the lifeguard helps you

Italian often uses the present tense in both parts of a real or general condition.

• Se non sai nuotare, il bagnino ti aiuta.

This expresses a general fact or usual result: whenever that condition is true, this is what happens.

English often does the same:

• If you can’t swim, the lifeguard helps you.

So this is a normal real condition pattern in Italian:

• se + present, present

Example:

• Se hai fame, mangi. = If you’re hungry, you eat.

The comma separates the if-clause from the main clause:

• Se non sai nuotare, = if you don’t know how to swim
• il bagnino ti aiuta. = the lifeguard helps you

When the conditional part comes first, a comma is very common and natural in writing.

A useful pronunciation note is the gn in bagnino.

• gn in Italian sounds like the ny sound in canyon
• so bagnino sounds roughly like ba-nyi-no

Also:

• nuotare begins with nuo-, pronounced roughly nwaw- or nwo-
• sai sounds like sigh

A rough pronunciation of the whole sentence is:

Se non sai nuotare, il bagnino ti aiuta
≈ seh non sigh nwee-oh-TA-reh, eel ban-YEE-no tee eye-YOO-tah

This is only approximate, but it can help you get started.