Learn Icelandic: Hún var svo þreytt að hún missteig sig og datt. - She was so tired that she took a bad step and fell.

The base verb is stíga (to step). The prefix mis- usually adds the meaning of doing something wrongly, badly, or by mistake. So, misstíga means to step badly or make a false step. Missteig is the past tense.

Why is the reflexive pronoun sig used with missteig?

In Icelandic, the verb misstíga is used reflexively (misstíga sig) to mean stumbling, taking a bad step, or even spraining an ankle. You can think of it literally as her mis-stepping herself.

Datt means fell, but what is its infinitive form and why does it look so different?

The infinitive is detta (to fall). It is a strong verb, which means it forms its past tense with a vowel change rather than a regular ending like -ði or -ti. Datt is the third-person singular past tense.

Is the structure svo ... að used exactly like the equivalent English phrase?

Yes! Svo followed by an adjective and then að works exactly like the English phrase so ... that to show cause and effect. Here, it connects þreytt (tired) to the result of her falling.