I have found that children have difficulty hearing the difference between f and th. This is a worksheet with similar sounding words, with the only difference is f or th.
To remind teachers and students, "/TH/" is said with your tongue in between your top and bottom teeth with the air passing through your teeth. Your tongue might feel funny from vibrating after holding /th/ out. "/F/" is said with your teeth biting your bottom lip, with the air going through your top teeth and lip. Your tongue is not involved.