Learn Icelandic: Það sem við þurfum að stoppa er mengunin.

In Icelandic, hvað is primarily reserved for actual questions. When 'what' means 'the thing that' or 'that which', you must use það sem. This structure is a pseudo-cleft sentence, which allows you to put heavy, specific emphasis on the thing being discussed—in this case, the pollution.

Since we are stopping the pollution, why is mengunin in the nominative case instead of the accusative mengunina?

If you wrote a standard sentence, 'We must stop the pollution' (Við þurfum að stoppa mengunina), it would indeed be accusative as the direct object of stoppa. However, in this structure, the whole phrase Það sem við þurfum að stoppa acts as the subject. The verb er links this subject to mengunin. Because the verb vera (to be) always equates two things using the nominative case on both sides, mengunin must be nominative.

Could we use the verb hætta instead of stoppa to mean 'stop' here?

No, stoppa is the right choice here. Hætta generally means to quit or cease an action you are doing yourself (like quitting a habit) and it takes a dative object. Stoppa is used when you are actively bringing an external process or physical thing to a halt, which perfectly fits the context of stopping pollution.

Why does Icelandic use the definite form mengunin when English often just says 'is pollution' without 'the'?