It has to be Y and S.The smtwtf are days of the week.The others are 4-3-4-3-4 letters apart in the alphabet
I and U are missing from the Elizabethan alphabet.
f and j
call a mechanic
There is one called I think missing letters but it is a puzzle you have to pick the right letter that will complete the two words. It gets harder as you get more right. Add me! I'm purplesquirtle
Multiplication
W and X
jj
MF, as in Mother F*****
It's likely that the missing letters are ff. They are the first letters of the counting numbers one, two, three, four, five, six, seven...
vacuum and continuum
Between A and D are two letters, between D and E are no letters, between E and H are two letters, and between H and I are no letters. Therefore, there would be two letters between I and the next letter, so it would be L.
Mj, ja