Are you focusing enough on learning synonyms of the words you already know??
Synonyms are words with similar meaning. Giving importance to synonyms matters a lot especially for the ESL learners.
Learning synonyms is a cool thing to do to expand our vocabulary. It was just a random idea that came to my mind a few months ago and which I ended up following later. I have the tendency of testing an idea myself first before recommending others to follow such ideas. So, I myself experimented with it for the first 3 weeks and the result was awesome.
Using synonyms can help a lot in increasing our vocabulary and also in improving conversational skills. Many a time, it happened to me that I ran out of proper words suiting a particular situation. It was not like I did not have words to express what I wanted to express. But those words were just not proper or at least would have been nasty if I had used.
Words, you use with your friends may be unsuitable in formal occasions. Words you use on formal occasions may be unsuitable for informal occasions too. It is not only about formal vs informal occasions. Its also about the context in which one is talking. Words suitable in a context might not be suitable in another.
Winding up, synonyms can make a person versatile speaker and writer as far as vocabulary and use of the alternative words is concerned. So next time, you look up a word in the dictionary, it would be wise to look up its synonyms too.