Learn Spanish (Spain): Quiero que estires los brazos ahora.

When you express a desire for someone else to do something, Spanish requires the subjunctive mood. The phrase Quiero que (I want that) is a classic trigger. Because the subject changes from 'I' wanting to 'you' doing the action, the regular present tense estiras becomes the present subjunctive estires.

Why does this sentence use the infinitive translation 'to stretch' in English, but a conjugated verb in Spanish?

If you want to do the action yourself, Spanish uses the infinitive: Quiero estirar means 'I want to stretch'. However, when you want someone else to do the action, Spanish breaks it into two parts separated by que, literally translating to 'I want that you stretch'.

Why does the Spanish use los brazos (the arms) instead of tus brazos (your arms)?

English uses possessive words like 'your' or 'my' for body parts, but Spanish strongly prefers definite articles like el, la, los, or las. Because the verb estires already shows that 'you' are doing the action, it is obvious whose arms are being stretched. Saying tus brazos here would sound unnatural.

Where is the word for 'you' in the Spanish sentence?

The 'you' is built directly into the verb ending of estires. You don't need a separate word for it, though you could add the pronoun tú for emphasis by saying Quiero que tú estires los brazos ahora.