Here, then, are 10 common mistakes that English speakers commonly make when they are learning Spanish. They aren't necessarily the most common errors, but they are ones that should studiously be avoided if you hope to get beyond a beginner's level.
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 usually means "pregnant" rather than "embarrassed," and that a violador usually is a rapist, not someone who merely committed a traffic infraction.
2. Using pronouns unnecessarily: With very few exceptions, English sentences require a subject. But in Spanish, that frequently isn't true. Where it would be understood by the context, the subject of a sentence (which in English often would be a pronoun) can and usually should be omitted. It usually wouldn't be grammatically incorrect to include the pronoun, but use of the pronoun can sound clunky or give it unnecessary attention.
3. Not learning how to use prepositions properly: Prepositions can 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. Always following English sentence order: You can usually follow English sentence order (except for putting most adjectives after the nouns they modify) and be understood. But as you're learning the language, pay attention to the many times where the subject is placed after the verb. Changing the word order can sometimes subtly change the meaning of a sentence, and your use of the language can be enriched as you learn different word orders. Also, some English constructions, such as placing a preposition at the end of a sentence, should not be imitated in Spanish.