Questions & Answers about Wenn er nervös ist, verrechnet er sich immer.
How does the prefix ver- change the meaning of rechnen in this sentence?
The base verb rechnen means "to calculate." As we are learning, the inseparable prefix ver- often indicates making a mistake. Therefore, sich verrechnen means "to miscalculate" or to make an error in your calculations.
Why is the reflexive pronoun sich used here?
Many verbs that use ver- to express a mistake require a reflexive pronoun. The infinitive is sich verrechnen. Because the subject is er (he), the matching reflexive pronoun is sich. You can literally think of it as him "miscalculating himself."
Why does the second half of the sentence start with verrechnet instead of er?
The first part of the sentence, Wenn er nervös ist, is a subordinate clause that acts as a single block taking up the first position. Since German sentences strictly place the main verb in the second position, verrechnet must come right after the comma, pushing the subject er to the third position.
Does wenn mean "if" or "whenever" here?
It actually means both! In German, wenn is used for conditional statements ("if") as well as repeated events in the present or future ("whenever"). Here, the word immer (always) strongly points to the "whenever" meaning.