"I have my letter."
Translation:Ik heb mijn brief.
Brief-brieven. Do nouns ending in -f need to change to -ven in plural?
This is a standard spelling rule indeed, but it's actually the other way around. The letters v and z are never at the end of a syllable and thus never at the end of a word. They are instead replaced by f and s respectively, so brief and brieven. The same principle is applied to the -z and -s, e.g. ik lees, wij lezen.
See this post for a more extensive explanation on Dutch spelling: https://www.duolingo.com/comment/3733043