Learn Icelandic: Við skulfum af spennu þegar leikurinn byrjaði. - We trembled with excitement when the game began.

English uses 'with' for many different things, but Icelandic is more specific. You use með for accompaniment or tools, but you use af to show the cause of a physical or emotional state. The excitement is what caused the trembling. Because af takes the dative case, the feminine noun spenna becomes spennu.

The verb skulfum looks unusual. What is its infinitive form?

It comes from the strong verb skjálfa, which means to shake, shiver, or tremble. Like many strong verbs, it changes its root vowel in the past tense: ég skalf (I trembled) becomes við skulfum (we trembled) in the plural.

Is spenna always a positive kind of excitement?

Not necessarily. Spenna translates literally to 'tension.' Depending on the context, it can mean thrilling excitement (like before a sports match), suspense in a movie, or stressful psychological tension.

I often mix up þegar and hvenær. Why is þegar the right choice here?

Hvenær is strictly a question word used to ask about time (for example, 'When does the game begin?'). You must use þegar as a conjunction to link two statements together, showing that one event happened at the time of another.