This is really starting to annoy me. However many times I listen, I can't hear any difference between f and th at all. So 'three' and 'free' and 'thought' and 'fought' sound -exactly- the same to me, and apparently I will always say it as 'free' and 'fought'. Can someone explain the difference without using any linguistic terminology (LOL, language student)? Like, what I should be doing with my mouth to make the 'th' in think/three/bath? Kind of sick of people making fun of me about it.

'ph' is just a regular 'f' though, right? and 'th' in, say, 'mother', is a 'v'?