1) Studying the words without real understanding When you merely memorize the new words, without really understanding them, you are actually studying "like a parrot".
In other words, you cannot really apply what you have learned.
This kind of misses the whole purpose, doesn't it? Before making exercises and activities with any new word - make sure you get its meaning.
That way you will have much better results.
2) Not practicing enough, or even worse: not practicing at all! Understanding a word's meaning is a great start, but if you want to be able to recognize and use it later, there is one thing you must do.
You must practice, practice, practice! Make up sentences, read stories, use flashcards, and so forth.
Take a moment and recall something you are really good at.
Have you practiced? Well, it's the same with building a vocabulary.
The more you use it, the more it is yours.
3) Learning difficult words before the basic ones Now, what would you say if I sent someone who is just starting to ski to the Olympics? Not a very good idea, is it? It is the same for vocabulary building.
If you are just starting out - begin with the basic words.
If you are more advanced - take it higher.
Start at your own level and advance step by step.
Otherwise, you might get confused and you might become overwhelmed.
Solve this by taking a gradual approach.
4) Not repeating the words you learned in the past If you only concentrate on new words, and do not refresh the "old" ones you have studied, you might be hurting your vocabulary building efforts.
New words learned need further repeating in future lessons.
The more you practice - the more it is yours.
Keep using the words and you are much less likely to forget them.
5) Learning in order to pass an exam, and for that reason only Some people study because they want to use the data.
In other words, they truly want to improve their vocabulary because they see the benefit they get from it.
Some people do it only to pass an exam or get some other "piece of paper".
They might indeed get a nice score, but that would probably be the only thing they get from it.
Exams and tests are there to verify your knowledge.
They are not the end product.
They can get you some nice pieces of paper, but you will not become a more skilled person because of your score.
Find out what is your real purpose and always keep it in mind.