Learn Swedish: Jag måste spela med och låtsas tro på bluffen. - I have to play along and pretend to believe in the scam.

The verb låtsas (to pretend) is special because it can take a bare infinitive directly, without needing att (to). You just say låtsas tro (pretend to believe), much like how modal verbs work. You could also use an att-clause if there were a new subject, like låtsas att jag tror (pretend that I believe), but when referring to the same subject, the bare infinitive is standard.

Since it follows the modal verb måste, shouldn't låtsas be in the infinitive form? Why does it end in an -s?

It actually is in the infinitive! Låtsas is a deponent verb, meaning it always ends in an -s in all of its forms, even though it has an active meaning. Its infinitive, present tense, and imperative forms all look exactly the same: låtsas. Here, both spela and låtsas are infinitives sharing the modal verb måste.

Is spela med a particle verb?

Yes, spela med translates to play along and functions as a particle verb here. This means the word med acts as a particle rather than a standard preposition, so it gets the primary spoken stress in the phrase: spela MED.

Why is it tro på bluffen instead of just tro bluffen?