Eu canso-me quando corro muito.

Because the verb here is cansar-se, which means to get tired or to become tired.

So:

• cansar = to tire someone/something out
• cansar-se = to get tired

In this sentence, me means myself, so canso-me literally means something like I tire myself / I get tired.

This is very common in Portuguese: some verbs are used with a reflexive pronoun even when English does not use one.

In European Portuguese, attaching the pronoun after the verb with a hyphen is very common in normal affirmative main clauses:

• canso-me
• levanto-me
• chamo-me

This is called enclisis.

In Brazilian Portuguese, me canso is much more common.
So if you are learning Portuguese from Portugal, canso-me is the expected form here.

However, pronoun position can change in other contexts. For example:

• Não me canso = I do not get tired

After não, the pronoun usually comes before the verb.

No, it is not strictly necessary.

Portuguese verbs usually show clearly who the subject is, so you can simply say:

• Canso-me quando corro muito.

The -o ending in canso and corro already shows that the subject is I.

Including Eu can add:

• emphasis
• contrast
• clarity

For example:

• Eu canso-me, mas ele não. = I get tired, but he doesn’t.

So in this sentence, Eu is optional.

Both are in the present indicative.

• eu canso-me = I get tired / I do get tired
• eu corro = I run

In Portuguese, the present tense is often used for:

• general truths
• habits
• things that happen regularly

So this sentence means something like:

• whenever I run a lot, I get tired
• I get tired when I run a lot

It is not necessarily about what is happening right now at this exact moment.

Because correr is the infinitive form, meaning to run.

After the subject eu, you need the conjugated form:

• eu corro
• tu corres
• ele/ela corre
• nós corremos
• vós correis
• eles/elas correm

So:

• quando corro muito = when I run a lot

not:

• quando correr muito in this sentence

Quando means when.

It introduces a time clause:

• quando corro muito = when I run a lot

So the structure is:

• main clause: Eu canso-me
• time clause: quando corro muito

You can also reverse the order:

• Quando corro muito, canso-me.

That means the same thing.

Here muito means a lot.

It modifies the verb corro, so:

• corro muito = I run a lot

This is different from using muito with a noun or adjective.

Examples:

• corro muito = I run a lot
• muita água = a lot of water
• muito cansado = very tired

So in your sentence, muito is an adverb, which is why it stays in that form and comes after the verb.

Yes, often you can, but the nuance is slightly different.

• Canso-me = I get tired
• Fico cansado = I become tired / I end up tired

Both are natural, but cansar-se is a very direct verb for getting tired.

Compare:

• Eu canso-me quando corro muito.
• Eu fico cansado quando corro muito.

Both are correct and natural.
The first uses the reflexive verb cansar-se.
The second uses ficar + adjective.

Because languages do not always build the same idea in the same way.

Portuguese often uses a reflexive form where English uses:

• get + adjective
• a non-reflexive verb
• a completely different structure

So cansar-se is just the normal Portuguese way to express this idea.

This does not always mean the action is truly reflexive in the literal English sense. It is simply how the verb works.

A useful contrast is:

• O exercício cansa-me. = The exercise tires me.
• Eu canso-me. = I get tired.

In the first sentence, something tires me.
In the second, I become tired.

Yes:

• Quando corro muito, canso-me.

This is very natural.

Portuguese allows flexible word order, especially with clauses like quando....
The meaning stays the same.

A comma is usually used when the quando clause comes first:

• Quando corro muito, canso-me.

But if the main clause comes first, no comma is usually needed:

• Canso-me quando corro muito.

It would be:

• Eu não me canso quando corro muito.

Notice the important change:

• affirmative: canso-me
• negative: não me canso

After não, the pronoun normally moves before the verb in European Portuguese.

This is a very important pattern to learn with reflexive and object pronouns.

No. It can also be used for mental or emotional tiredness, depending on context.

For example:

• Canso-me quando corro muito. = physical tiredness
• Canso-me de esperar. = I get tired of waiting
• Canso-me destas discussões. = I get tired of these arguments

So cansar-se can mean:

• to get physically tired
• to get fed up
• to grow tired of something

In your sentence, because of corro muito, the meaning is clearly physical.