Learn Swedish: Kollegan påstod att lögnen var farlig. - The colleague claimed that the lie was dangerous.

This happens because of "backshifting" in indirect speech, which is the main focus of this lesson. Since the reporting verb påstod (claimed) is in the past tense, the verb in the att-clause shifts to the past tense too. Even if the colleague's original exact words were "Lögnen är farlig" (The lie is dangerous), the är shifts to var to match the past-tense perspective of påstod.

How do I remember the past tense form påstod?

Look at the root of the word! Compound verbs in Swedish almost always conjugate exactly like their base verb. Påstå (to claim) ends with stå (to stand). Since stå becomes stod in the past tense, påstå simply becomes påstod.

I often confuse words ending in -an. Is kollegan singular or plural here?

Kollegan is singular (the colleague). For en-words that end in -a (like en kollega), you simply add an -n to make the definite form. It is very common to mix this up with plurals, but the plural "the colleagues" would be kollegorna.

Could I use att ljuga (to lie) instead of lögnen (the lie) here?