Learn Swedish: Misstanken mot arvingen måste ha varit fel. - The suspicion against the heir must have been wrong.

Just like in English, the modal verb måste (must) isn't only for rules or obligations. It is also used for logical deductions. When combined with ha and the supine form of a verb (here, varit), it expresses strong certainty about a past event, translating perfectly to "must have been".

Why do we use the preposition mot for "against" here instead of emot?

Mot is the standard preposition to use right before a noun to mean "against" or "towards" (as in mot arvingen). Emot is mostly used as a verbal particle (like in ta emot, meaning to receive) or placed independently as an adverb without a following noun.

Both misstanken and arvingen end in -en. What are their base (indefinite) forms?

They both come from en-words that already end in -e: en misstanke (a suspicion) and en arvinge (an heir). For nouns like these, you don't add the full -en suffix; you simply add an -n to make them definite, resulting in misstanken and arvingen.

Misstanken is an en-word, so why doesn't the adjective fel take any special ending?