Learn Icelandic: Við lentum í mikilli umferð á leiðinni.

The phrase að lenda í literally means "to land in," but it is a very common idiom meaning "to get caught in," "to end up in," or "to experience." It is often used for unexpected or unpleasant situations, like getting stuck in traffic, getting into debt, or having an accident.

Why is it mikilli umferð instead of the nominative mikil umferð?

The phrase lenda í requires the following noun to be in the dative case. Since umferð (traffic) is a feminine word, both the adjective mikill and the noun change to their dative feminine forms: mikilli umferð.

We say "heavy traffic" in English. Does mikill mean "heavy"?

Not literally. Mikill generally means "much," "great," or "a lot of." While English describes bad traffic as "heavy," Icelandic describes it as "much" traffic. If you tried to use the literal Icelandic word for heavy (þungur), it would sound strange here.

How does the word leiðinni break down in the phrase á leiðinni?

It is the noun leið (way or route) plus the definite article suffix -inni (the). The preposition á takes the dative case here to show location, so the feminine dative form of the word with the article becomes leiðinni. The whole phrase á leiðinni perfectly matches the English "on the way."