Why did I hear a hint of "r" in muizen? Is that how it's pronounced? I heard muirzen.
I heard it too and couldn't figure out what that word was. Now that I know it's muizen, I can't really hear the r.
Read the notes beneath the lessons (in the menu, Always do that, the early ones are critical). You must maintain stresses and sometimes syllables (adding on) in the plural.
'S' would create mui-Sen (hyphen not written) instead of Muizen and Muis (s is not emphasized).