You can easily come off as an intelligent person while talking, but as soon as you have to type the English language, you can come to an unfortunate problem: English grammar can be hard to learn. It's necessary to learn, and with some practice, you can master the English arts.
Contractions are hard to get down for some. Perhaps one of the most common mistakes is the use of "your" and "you're." When you see the contraction "you're" you should be able to replace it with "you are" and the sentence should make sense still. "Your" is instead used to show ownership of an item. An example could be "She said you're welcome to have a cookie, so make your decision." As we can see, the sentence makes sense when using "you are" in place of contractions.
Probably the second most common mistake in grammar is "there" being confused with "their." The word "their" shows possession, while "there" is indicating a location. If your neighbors own a pool, you would say "The neighbors often swim in their pool," and use there as "Sometimes I swim over there."
Sometimes even the professionals can get tripped up on the proper usage of some words. "Effect" and "affect" is a pair of words that tend to do this. When you use the two words, always remember that "affect" is going to be a verb, and it will do action in a sentence, such as "Germs affect health." On the other hand we have "effect," which is a noun or adjective, and is used in a case such as "The effect of being sick is missing school or work."
When those who are foreign to the English language start to make their efforts in learning it, the biggest stumbling block is when to use "a" and when to use "an." The word "an" is used before a word that sounds like a vowel- which seems easy enough to see at first. The problem is that some words such as "hour" don't appear to require the word "an" until it is pronounced. It takes a lot of practice, but practice does make perfect.
"Good" and "well" are taught incorrectly in common United States slang. It's common to say "I am feeling good," but this is actually not correct. "Good" is not a noun, but instead an adjective. "Well" is a noun, and would instead take the place of good in this case. "I am/am not feeling well" should be the proper response if asked how you are feeling.
Final Thoughts
Keep your nose to the grindstone, so to speak, and your attempt at grasping the English language will lead to success. If you decide you need help, there are many sources over the Internet and tutors you can find in your locale to learn further.