Learn Italian: Il nostro passatempo era giocare nel cortile.

Era is the imperfetto form of essere (to be) for he, she, or it. Essere is irregular, so it doesn't follow the normal past tense rules for -are, -ere, or -ire verbs. We use the imperfetto here because it describes an ongoing state or habit in the past, setting the scene rather than talking about a single completed action.

Why does the Italian sentence use the infinitive giocare instead of the -ing form giocando for "playing"?

In English, we often use the -ing form as a noun (like "playing is fun" or "my hobby is playing"). In Italian, you must use the infinitive (the basic "to do" form) when a verb acts as a noun. So, this sentence literally translates closer to "Our pastime was to play."

Why is there an Il before nostro? Doesn't nostro already mean "our"?

Yes, but in Italian, possessive words like mio, tuo, and nostro almost always require a definite article before them. The main exception is when you are talking about singular family members (like nostro padre). Since passatempo is a regular noun, you must include the article to say il nostro passatempo.

How do you get nel for "in the"?