How to correct my pronunciation

hello guys!!

i am Kevin,ive been practicing my pronunciation for months now but it seems that i am still not improving.cuz every time i have a conversation with other people i always forgot to pronounce the right words..but when i am reading and seeing the words on the textbook i can able to pronounce the word correctly..i had this situation before when i was talking my teacher about how to find the program on the computer.. and when i said "it can be found on the lower left of the screen.." what she understood was she can "pound" the screen:-D.. that was the time i realize the i am very poor pronouncing words with TH V&B F&P sounds..

can somebody help me

What is your nationality? I suspect you are Filipino. The 'p' for 'f' substitution is very common with native Tagalog speakers.

The difference you have missed between the two pronunciations is this: when saying a 'p' you tightly close your lips and let the air escape abruptly with a popping sound. The teeth are held apart and are not used at all in pronouncing 'p.' When pronouncing an 'f' the lips are held slightly apart, with the upper front teeth lightly touching the lower lip. The air is not popped out, it is more gently passed out of the mouth on either side of the front teeth.

In ancient Egyptian the symbol for the 'f' sound was a snake. Think of a snake hissing, and then you understand that the 'f' sound is not popped like a 'p' but hissed out, rather like the sound a snake makes.

The sounds for 'v' and 'b' are often difficult to differentiate for native Spanish and Russian speakers. Since I am operating on the assumption that you are Filipino, I suspect this difficulty arises for you from a Spanish origin. It is the same distinction as above: 'b' is pronounced with the lips together and the air popped out; 'v' is pronounced with the upper front teeth pressing the lower lip and, in this case, it is hummed out from the back of the throat.

The diphthong 'th' is difficult for many, even native English speakers from Ireland. In ancient Greek, there were two distinct sounds for 'th' one of which is not used in English. In English, 'th' is pronounced by placing the tip of the tongue on the edge of the upper front teeth, and arching it downward in the mouth. Then air is hissed out on either side of the tongue. It is a hissing sound, not a popping sound, and the lips never come together. There must be a large space between the tongue and the roof of the mouth in order to pronounce 'th' properly, hence the instruction to arch the tongue downward from the roof of the mouth while keeping the tip of the tongue firmly against the very edge of the upper front teeth.

Aren't there two pronunciations of th in English- /ð/ (voiced) & /θ/ (unvoiced)?

Also, is th really a diphthong?

Most definitions define it as two vowel sounds said as one, a vowel and a glide, or a vowel whose quality changes:

diphthong noun - definition in British English Dictionary & Thesaurus - Cambridge Dictionary Online
diphthong - Dictionary definition and pronunciation - Yahoo! Education
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What is a diphthong?

I don't see how th could be described as a diphthong from any of those definitions.