Learn French: Ma sœur adorait cette vieille poupée. - My sister used to love this old doll.

This is the imperfect tense. To build it, you start with the present tense nous form of the verb adorer, which is nous adorons. You drop the -ons to get the stem (ador-), and then add the imperfect ending for the 3rd person singular, which is -ait.

Why does the English translation say "used to love" instead of just "loved"?

The imperfect tense is used for ongoing states, feelings, or past habits. While you could translate it simply as "loved", saying "used to love" perfectly highlights that your sister's love for the doll was a continuous state in the past, rather than a single completed action.

I thought the French word for "old" was vieux. Why is it vieille here?

Vieux is the masculine singular form. Because poupée (doll) is a feminine noun, the adjective describing it must be feminine too. The feminine form of vieux is the irregular word vieille.

Why does vieille come before the noun? Don't most French adjectives come after?

Most do! However, a small group of very common, short adjectives go before the noun. You can remember them with the acronym BANGS, which stands for Beauty, Age, Number, Goodness, and Size. Since vieille describes Age, it is placed before poupée.