A professora diz que, se mexermos mal o rato, o pulso e o cotovelo cansam-se depressa.

In Portuguese, it is very common to use the definite article before a noun when talking about a specific person or role: a professora = the teacher.

European Portuguese often uses the article in places where English would not. So A professora diz... is a perfectly natural way to say The teacher says... or sometimes just Teacher says... depending on context.

Without the article, professora would usually sound less complete here.

Que introduces a subordinate clause, like that in English:

• A professora diz que... = The teacher says that...

In English, that is often omitted:

• The teacher says if we...

In Portuguese, que is normally kept in this kind of sentence.

That part is an inserted conditional clause:

• A professora diz que, se mexermos mal o rato, o pulso e o cotovelo cansam-se depressa.

The commas help separate the if clause from the main clause. English does something similar:

• The teacher says that, if we move the mouse badly, the wrist and elbow get tired quickly.

You may sometimes see slightly different punctuation styles, but these commas are normal and helpful.

After se meaning if, Portuguese often uses the future subjunctive when talking about something possible in the future or in a general condition.

So:

• se mexermos = if we move / if we happen to move
• not se mexemos

Mexemos is the present indicative, and it would not be the normal choice in this structure.

This is a very important pattern in Portuguese:

• se eu fizer = if I do
• se tu fores = if you go
• se nós mexermos = if we move

Good question: the form mexermos can look identical to the personal infinitive.

Here, it is future subjunctive, because it comes after se in a real conditional clause:

• se mexermos mal o rato = if we move the mouse badly

Why it is not the personal infinitive here:

• the personal infinitive usually appears after prepositions or certain structures, such as para mexermos (for us to move), antes de mexermos (before we move)

So the form is the same, but the grammar function is different.

Here rato means computer mouse, not the animal.

And mexer o rato means something like:

• move the mouse
• handle/use the mouse

The verb mexer is very broad. It can mean move, touch, stir, fiddle with, depending on context.

In computer-related language, mexer no rato or mexer o rato can refer to moving or using the mouse. In this sentence, it clearly means moving/handling the computer mouse incorrectly.

Because mal is an adverb, while mau is an adjective.

Here it describes how we move the mouse:

• mexer mal = to move badly / incorrectly

Compare:

• um rato mau = a bad mouse (adjective describing the noun)
• mexer mal o rato = to move the mouse badly (adverb describing the verb)

This is a very common distinction:

• bom / bem
• mau / mal

Because the subject is plural:

• o pulso e o cotovelo = the wrist and the elbow

Two singular nouns joined by e normally make a plural subject, so the verb must also be plural:

• o pulso e o cotovelo cansam-se

If there were only one subject, it would be singular:

• o pulso cansa-se
• o cotovelo cansa-se

In this sentence, cansar-se means to get tired.

So:

• cansam-se = they get tired

This is a very common pronominal use in Portuguese:

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

Compare:

• O exercício cansa o braço. = The exercise tires the arm.
• O braço cansa-se. = The arm gets tired.

So the se is part of the verb expression cansar-se.

Portuguese often uses the singular for body parts in general statements, especially when talking about what happens to a person’s body in a typical situation.

So:

• o pulso e o cotovelo cansam-se depressa

can sound more natural than making everything plural, depending on the intended meaning.

If the sentence were talking about people in general as a group, you might also hear plural forms in other contexts, but the singular here is very natural because it refers to the wrist and elbow as body parts affected during the action.

Portuguese often uses we in a general or inclusive way, just like English can:

• If we sit badly, our back hurts.
• If we sleep too little, we get tired.

Here mexermos does not necessarily mean only the speaker and another person. It can mean we people / we students / anyone in this situation.

So it is an inclusive, general we.

Depressa means quickly or soon in this context.

So:

• cansam-se depressa = get tired quickly

It is very common in everyday Portuguese. A near equivalent is rapidamente, but depressa is often more natural in ordinary speech.

Compare:

• Ele fala depressa. = He speaks quickly.
• Fiquei cansado depressa. = I got tired quickly.

Yes. Rapidamente would be grammatically correct:

• ...o pulso e o cotovelo cansam-se rapidamente.

But depressa is shorter and very natural in speech and everyday writing. Rapidamente can sound a bit more formal or more neutral depending on context.

So both work, but depressa is an excellent everyday choice.

Yes. Just like English mouse, Portuguese rato can mean:

• the animal: a rat / mouse depending on context
• the computer device: mouse

In a sentence about moving it badly and causing wrist/elbow strain, the meaning is clearly computer mouse. Context removes the ambiguity.