Learn Polish: Ponieważ był chory, nie był w stanie krzyknąć. - Since he was sick, he wasn't able to shout.

Krzyczeć is imperfective and means to shout in general or continuously. Krzyknąć is a semelfactive perfective verb created with the -nąć suffix. It means to let out a single, instantaneous shout or cry. Using krzyknąć here emphasizes that he couldn't muster the energy for even one single yell.

How does the stem change when forming krzyknąć from krzyczeć?

When adding the -nąć suffix to form a single-action verb, the stem often simplifies. The -cze- part of the imperfective krzyczeć drops away, revealing the base root krzyk (which also happens to be the noun for a shout). Then you simply attach -nąć.

Could I just use bo instead of ponieważ at the beginning of this sentence?

It is much better to use ponieważ here. While both mean because, ponieważ is the standard, natural choice for starting a sentence, acting much like the English word since. Bo is more conversational and is almost exclusively used in the middle of a sentence to link a second clause to the first.

Is there a difference in meaning between nie był w stanie and nie mógł?