Learn Swedish: Jag har tjocka strumpor.

Because it is describing a plural noun (strumpor). When an adjective describes a plural noun in Swedish, it takes the plural form, which almost always means adding an -a to the base form. The singular word for thick is tjock, so the plural becomes tjocka.

What is the singular form of strumpor?

The singular is en strumpa (a sock). In Swedish, en-words that end in -a usually form their plural by dropping the -a and adding -or.

Unlike English, do Swedish adjectives always change when the noun is plural?

Yes! In English we just say thick socks (the adjective itself doesn't change). In Swedish, attributive adjectives must agree with the noun. Since strumpor is plural, the adjective must also be in its plural form (tjocka).

How do I pronounce the tj in tjocka?

The tj spelling represents a specific Swedish sound. It sounds similar to a soft English sh or the ch in choose, depending on the regional accent. You might remember this sound from other common words like tjugo (twenty) or tjej (girl).