Learn Icelandic: Hann kláraði maraþonið þrátt fyrir meiðslin. - He finished the marathon despite the injuries.

The compound preposition þrátt fyrir always requires the accusative case. Because meiðsl (or meiðsli) is a neuter noun, its accusative form is the same as its nominative form, becoming meiðslin when the plural definite article is attached.

Is meiðslin singular or plural here?

It is plural. The word meiðsl is a neuter noun that is almost exclusively used in the plural form in Icelandic when referring to physical injuries. You can tell it is plural here because the neuter definite article is -in in the plural, whereas it would be -ið in the singular.

Could I use þótt instead of þrátt fyrir in this exact sentence?

Not directly. While both convey a similar meaning, þrátt fyrir is a preposition that takes a noun phrase like meiðslin. The word þótt (although) is a conjunction and requires a full subclause with a verb, such as þótt hann væri meiddur (although he was injured).

What does the word þrátt mean on its own?

Outside of this phrase, þrátt is the neuter form of the adjective þrár, which means stubborn or obstinate. The prepositional phrase þrátt fyrir literally carries the feeling of standing stubbornly against an obstacle!