Learn Swedish: Han halkade på isen och höll på att stuka foten. - He slipped on the ice and almost sprained his foot.

While håller på att usually creates a continuous tense (like "is doing"), using it in the past tense (höll på att) with a sudden, accidental action like stuka (sprain) changes the meaning. Instead of meaning he was in the process of spraining it, it means he was right on the verge of doing it, which we translate as "almost".

Could I just use the word nästan and say och stukade nästan foten instead?

Yes, that is grammatically correct and means the same thing. However, using höll på att is extremely natural and common in Swedish for accidents and narrowly avoided events, as it vividly emphasizes being on the absolute brink of something happening.

Why is it foten (the foot) and not a possessive like hans fot or sin fot?

In Swedish, it is standard to use the definite form for body parts and clothing (like foten) when the owner is obvious from the context. Saying sin fot wouldn't be strictly wrong, but using the definite article foten sounds much more natural to a native speaker here.

How would höll på att stuka sound in fast, casual spoken Swedish?