Learn Swedish: Tröttheten är ett svårt problem. - Fatigue is a difficult problem.

It is built from the adjective trött (tired). By adding the suffix -het, it becomes the noun trötthet (tiredness or fatigue). As a rule, all nouns ending in -het are en-words. Finally, the -en at the very end makes it definite.

Why is tröttheten in the definite form if the English translation is just 'fatigue'?

In Swedish, abstract nouns or general concepts often take the definite form when you are talking about them as a whole. While English says 'Fatigue is...', Swedish naturally says Tröttheten är... (literally 'The fatigue is...').

Why is there a -t at the end of svårt?

The adjective svår (difficult or severe) describes the noun problem. Since problem is an ett-word (ett problem), the adjective must take a -t to match the neuter gender, becoming svårt.

Is there a trick to remember that problem is an ett-word?

Yes! problem is an ett-word (ett problem). A helpful B1 refresher is that many international loan words in Swedish that end in a consonant, such as system, program, and problem, are almost always ett-words.