Breakdown of Þjófurinn var að hlaupa út.
vera
to be
að
to (infinitive marker)
hlaupa
to run
út
out (motion outwards)
Questions & Answers about Þjófurinn var að hlaupa út.
How does the grammar for "was running" work in this sentence?
Just like in English, Icelandic can show that an action was ongoing in the past. It uses the past tense of vera (to be) plus að plus the infinitive verb. Here, var (was) plus að hlaupa (to run) creates "was running". If you just wanted to say the simple past "The thief ran", you would drop the að and just say Þjófurinn hljóp.
What is the base word for Þjófurinn?
The base word for "thief" is þjófur. In Icelandic, the word for "the" attaches to the end of the noun. Because þjófur is a masculine word and the subject of the sentence, it takes the definite article suffix -inn to become "the thief".
Could I use úti instead of út in this sentence?
No, because the thief is actively moving towards the outside. Icelandic strictly distinguishes between motion and location. You must use út (outwards) when there is movement, like running out. You would only use úti (outside) when someone or something is already located there and not changing location.