Learn Italian: Non gridare per un ragno, anch'io avevo paura! - Don't shout because of a spider, I was afraid too!

This is the negative imperative (a negative command) for tu (you, informal). To tell a friend or family member not to do something, you simply use non followed by the infinitive of the verb. So, the phrase for 'don't shout' is non gridare.

I thought per meant 'for'. Why does it mean 'because of' in this sentence?

While per usually translates to 'for', it is also commonly used in Italian to show the cause or reason for an action. In this context, per un ragno means 'because of a spider' or 'over a spider'.

Why is it written as anch'io instead of anche io?

When the word anche (also or too) comes right before a pronoun that starts with a vowel, like io (I), Italians usually drop the 'e' and add an apostrophe. This makes the two words flow together smoothly when speaking.

Why use the imperfect tense avevo instead of the passato prossimo?

The imperfect tense (avevo) is used here because it describes a past state of being or an ongoing feeling. The speaker is talking about a general feeling of fear they experienced in the past, rather than a single, isolated action. Also, remember that in Italian you 'have' fear (avere paura) rather than 'are' afraid.