FORVO is a great website because you can listen to any word in English and hear the word in different accents such as British, American, Texan, Australian, etc. 

Common Pronunciation Problems

Past participles ending in -ed eg walked, missed, visited
A general rule is that words ending ‘p’, ‘k’ or ‘s’ take the ‘t’ sound (stopped / missed / asked), words ending ‘l’, ‘m’ or ‘n’ take the ‘d’ sound (calmed / imagined / traveled) and those ending ‘t’ or ‘d’ take the ‘id’ sound. 

the "th" sound
There are two types of "th" pronunciation - hard and soft. Hard eg in "the" and soft eg in "think". 

L & R
Some students confuse these two sounds for example they say "flied lice" instead "fried rice". 

This and These
There is a difference between these two words. This has a hissing "s" sound at the end and "these" has more of "z" sound.
Also, these is pronounced like "theez". Compare the 2 here:

Homophones are words that sound the same but are spelt differently. 

This site has more pronunciation practice: