Minhas pernas ficam cansadas quando eu corro no parque.

Yes. Minhas means my and agrees with pernas.

Because pernas is:

• feminine
• plural

the possessive has to be feminine plural too:

• meu = my, masculine singular
• minha = my, feminine singular
• meus = my, masculine plural
• minhas = my, feminine plural

So:

• minha perna = my leg
• minhas pernas = my legs

Because the subject is minhas pernas, which is plural.

In Portuguese, the verb usually agrees with the subject:

• minha perna fica cansada = my leg gets tired
• minhas pernas ficam cansadas = my legs get tired

So ficam is the third person plural form of ficar.

Ficar + adjective often means to become, to get, or to end up being in a certain state.

So ficam cansadas means:

• get tired
• become tired

That is more natural here than using ser.

Useful comparison:

• ser cansado/a would suggest a more permanent characteristic, and it usually would not fit here
• estar cansado/a means to be tired
• ficar cansado/a means to get tired

So:

• Minhas pernas estão cansadas. = My legs are tired.
• Minhas pernas ficam cansadas quando eu corro. = My legs get tired when I run.

Because adjectives in Portuguese usually agree with the noun they describe.

Here, cansadas refers to pernas, which is:

• feminine
• plural

So the adjective must also be feminine plural:

• cansado = masculine singular
• cansada = feminine singular
• cansados = masculine plural
• cansadas = feminine plural

Examples:

• o braço cansado = the tired arm
• a perna cansada = the tired leg
• os braços cansados = the tired arms
• as pernas cansadas = the tired legs

Both are in the present indicative.

• ficam = they get / they become / they stay
• corro = I run

In this sentence, the present tense expresses a habitual or general truth:

• My legs get tired when I run in the park

It does not have to mean only right now. It can mean this usually happens.

Yes, Portuguese often drops the subject pronoun because the verb ending already shows the person.

So both are possible:

• quando eu corro no parque
• quando corro no parque

Both are natural.

Including eu can:

• add clarity
• add emphasis
• make the sentence sound a little more explicit

Since corro clearly means I run, eu is not required, but it is completely normal to include it.

Quando means when.

It introduces a time clause:

• quando eu corro no parque = when I run in the park

So the sentence has:

• main clause: Minhas pernas ficam cansadas
• subordinate time clause: quando eu corro no parque

You can also switch the order:

• Quando eu corro no parque, minhas pernas ficam cansadas.

That means the same thing.

Because no is a contraction of:

• em + o = no

So:

• no parque literally comes from in the park

This kind of contraction is very common in Portuguese:

• em + o = no
• em + a = na
• em + os = nos
• em + as = nas

Examples:

• no parque = in the park
• na escola = at/in the school
• nos Estados Unidos = in the United States
• nas ruas = in the streets

Yes. As minhas pernas is also correct.

Portuguese often allows a definite article before possessives:

• minhas pernas
• as minhas pernas

In Brazilian Portuguese, both are possible. The version without the article is very common and sounds natural here.

So:

• Minhas pernas ficam cansadas...
• As minhas pernas ficam cansadas...

Both work.

By itself, corro is the simple present, so it most naturally means:

• I run
• I do run

In context, it often expresses a habitual action:

• quando eu corro no parque = when I run in the park

If you specifically want I am running right now, Brazilian Portuguese often uses:

• estou correndo

So:

• eu corro no parque = I run in the park / I run at the park
• estou correndo no parque = I am running in the park

Because cansadas is not directly modifying the noun inside the noun phrase. It is part of the predicate after ficam.

Compare:

1. Minhas pernas cansadas doem.

   • cansadas directly describes pernas
   • literally: My tired legs hurt

2. Minhas pernas ficam cansadas.

   • cansadas is the result/state after ficam
   • literally: My legs get tired

So in your sentence, cansadas is a predicate adjective, which is why it comes after the verb.

Yes. Portuguese allows some flexibility.

These are both natural:

• Minhas pernas ficam cansadas quando eu corro no parque.
• Quando eu corro no parque, minhas pernas ficam cansadas.

The second version puts more focus on the time/situation first.

The meaning stays basically the same.

In Brazilian Portuguese, minhas is often pronounced roughly like:

• MEEN-yas
  or in many everyday pronunciations, something close to
• MIN-yas

The nh sound is important. It is like the ny sound in canyon.

So:

• mi-nhas is not pronounced with a plain English n
• it has that softer ny sound

A rough full pronunciation of the sentence could be:

MEE-nyas PER-nas FEE-kam kan-SA-das KWAN-do eh-o KOH-ho no PAR-kee

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

Yes, but the meaning changes slightly.

• Minhas pernas ficam cansadas quando eu corro no parque.

  • My legs get tired when I run in the park.
  • Focus on the change of state.

• Minhas pernas estão cansadas quando eu corro no parque.

  • This sounds less natural for a general statement.
  • It suggests they are tired during that situation, rather than become tired.

For a repeated cause-and-effect idea, ficam cansadas is the better choice.

Usually it sounds like a general habit or repeated experience.

Because it uses the present tense:

• ficam
• corro

the sentence normally means something like:

• whenever I run in the park, my legs get tired

If you wanted one specific past event, you would use past forms, for example:

• Minhas pernas ficaram cansadas quando eu corri no parque.
• My legs got tired when I ran in the park.