One online tool that is great for translating between Spanish and English is Spanishdict.com. This site aggregates three online translation tools to give you the best Spanish to English translation that you can get for free.
The translator will only function properly if you make sure to correctly use accent marks. Fortunately, the entry box has a toolbar that allows you to enter the necessary accents easily. Along with offering sentence translation, it gives a word-by-word breakdown of your entry in case the translations seem like they could be better worded. If the context is unconventional, using other online dictionaries, such as WordReference.com, help to give you the most accurate translation.