Learn Portuguese (Portugal): Por mais que eu treine, o surf é difícil. - No matter how much I train, surfing is difficult.

The phrase por mais que (meaning no matter how much or however much) sets up a concession—an action that doesn't change the final outcome. In Portuguese, this structure requires the subjunctive mood because it focuses on a hypothetical or indefinite degree of effort. That is why the verb treinar becomes treine here.

Could I use por muito que instead of por mais que in this sentence?

Yes! Por muito que eu treine means exactly the same thing. Both por mais que and por muito que are completely interchangeable in this context, and both require the following verb to be in the subjunctive.

Why is there an o before surf? In English, we just say surfing.

In Portuguese, you generally use definite articles (o or a) before the names of sports, activities, and general concepts when they act as the subject of the sentence. Since surf is the subject here, it requires the masculine article o.

Why use é instead of está for is difficult?

The verb ser (conjugated here as é) is used for inherent, defining characteristics. By using é, the speaker is saying that surfing is a fundamentally difficult sport by nature, rather than just temporarily difficult for them at this exact moment.