Learn Swedish: Vi måste förhöra de här misstänkta igen. - We must interrogate these suspects again.

In Swedish, you can often turn an adjective or a past participle into a noun to describe a group of people. Here, the past participle misstänkt (suspected) is used in the plural form misstänkta to mean the suspected ones or the suspects.

Since we are using de här (these), shouldn't the noun be in the definite form?

Yes, and it is! When you turn a participle into a noun, it follows adjective rules. For adjectives and past participles, the plural ending is -a, which is also the definite ending. So misstänkta is the correct definite plural form to follow de här.

How would I say 'this suspect' if there was only one?

You would use den här instead of de här. For the word suspect, you use den här misstänkta (for a female or a person whose gender is unknown) or den här misstänkte (specifically for a male). The -e ending is a special masculine definite form often used for people.

Why doesn't förhöra have an -r at the end?

It follows the modal verb måste (must). Just like in English where you say must interrogate rather than must interrogates, Swedish modal verbs are followed directly by the infinitive form of the next verb, which is förhöra.