1. Assuming that Spanish words that look like English words mean the same thing: Words that have the same or similar form in both languages are known as cognates. Since Spanish and English share a large vocabulary derived from Latin, more often than not words that are alike in both languages have similar meanings. But there are plenty of exceptions, and it wouldn't be a bad idea to study these lists of false cognates and partial cognates. You'll find, for example, that embarazada means "pregnant" rather than"embarrassed."
2. Not learning how to use prepositions properly: Prepositionscan be notoriously challenging. It can be helpful to think about the purpose of the prepositions as you learn them, rather than their translations. This will help you avoid mistakes such as pienso acerca de ti for "I'm thinking about you" instead of pienso en ti.
4. Not learning the subjunctive mood: In English, we seldom make a distinction when verbs are in the subjunctive mood. But the subjunctive can't be avoided in Spanish if you wish to do more than state simple facts and ask simple questions.
5. Being afraid to make mistakes: Mistakes are inevitable with learning, and the worst mistake you could make would be to be fearful of using what you know. Remember that no matter how many mistakes you make, wherever you go in the Spanish-speaking worldyour sincere attempts to learn the language will always be appreciated.
Source: Gerald Erichsen, About.com Guide