Learn Swedish: Kunden köper tjugo förpackningar pasta. - The customer buys twenty packages of pasta.

In Swedish, when expressing a quantity or measurement, you put the unit (förpackningar) directly next to the item being measured (pasta) without using a preposition. While English says 'packages of pasta', Swedish simply says 'packages pasta'.

Why is pasta in its indefinite form here?

In these types of quantity phrases, the substance being measured is almost always in the indefinite form. The customer is buying a general substance, not 'the' specific pasta (pastan).

How can I easily remember the gender and plural form of förpackningar?

The singular is en förpackning. A very reliable rule is that Swedish nouns ending in -ing are almost always en-words, and they regularly take the -ar ending to become plural.

Could I say paket instead of förpackningar in this sentence?

Yes, ett paket (a packet or package) is a very common alternative. If you used it, the phrase would be tjugo paket pasta, because paket is an ett-word that ends in a consonant, meaning its plural form looks exactly the same as its singular form.