Learn Icelandic: Við kynntumst í gær. - We got to know each other yesterday.

The base verb is kynna, which means "to introduce." By adding the -st ending to make it middle voice, it takes on a reciprocal meaning ("to each other"). So instead of introducing someone else, kynntumst means "we introduced ourselves to each other" or "we got to know each other."

How is the past tense built in kynntumst?

First, you conjugate the active verb kynna into the past tense for við (we), which gives you kynntum (we introduced). Then, you simply attach the -st ending directly to that past tense form to get kynntumst.

Why isn't there a separate word for "each other" in the Icelandic sentence?

The -st ending on middle voice verbs often does that job for you! Because kynntumst already implies a reciprocal action (doing something to each other), adding extra words for "each other" would be repetitive.

Why do we say í gær instead of just gær for "yesterday"?

In Icelandic, the concept of "yesterday" is always expressed as a prepositional phrase: í gær (literally "in yesterday"). You cannot use the word gær entirely on its own.