Jaden, you seem to be having difficulties with Homophones - the words that sound the same, but have different meanings. It's not a big deal, it's a common error for non-native English speakers. It just means you have to be extra careful because, if they are correctly spelled, they will NOT appear in your grammar check. I actually made a mistake like that in the latest chapter of my fic - I wrote 'debts' where I really wanted to use the word 'depths', but fortunately I have awesome readers who pointed it out for me. ^o^ You just have to be very mindful to check all the words you are not sure of.
Here are a few links that can help you:
Cambridge Advanced Learner's Dictionary Online
One of the BEST dictionaries you can find online.
Dictionary of same-sounding words
Okay, so the name is a bit clumsy, but this is a tremendously useful little thing and it is closely connected to your specific problem.
Finally, if all of the above fail you, I'm sure you will be able to find what you need here. I haven't been using it since I have the actual CALD with the CD which has an awesome Thesaurus included as a bonus, but I'm sure it's a great source.
Hope this helps!