Learners make mistakes and reinforce them because they produce sentences 1) too carelessly or 2) too early. You will avoid mistakes if you follow a couple of rules:

Rules of error-free speaking and writing

1. Use simple language. Some beginners try to build very complicated sentences with things like the present perfect tense or conditionals. They make horrible mistakes. Don't do this! If you've just started to speak or write in English, you should say what you can say (simple sentences that you have seen many times) - not what you want to say (complicated sentences). You may feel you're talking like a child or that you are not expressing your thoughts, but don't worry about it. Right now, your task is not to express your thoughts freely; your task is to learn the language.
2. Be slow and careful. In the beginning, you should write very slowly. If you need 2 hours to write an e-mail message with 10 correct sentences, that's okay. That's how long it should take if you're just starting to write.
   Why should it take so long? Because you should read your sentences many times, looking for mistakes. You should correct your own sentences frequently. You should check if your sentences are correct by using a dictionary and the Web. And you should look for example sentences to imitate.
   When you're speaking, it's okay to build a sentence for some time in your head before you open your mouth.
3. If you're not sure how to say something, don't say it. If you can't say something correctly, it's almost always better not to say it. You don't want to teach yourself the wrong way to say it. You can try to look for the correct sentence in a dictionary or on the Web (see next point), but when speaking, usually you don't have time for that. So it's a good idea to say something else - something that you know is correct. It can even be something on a different subject.
4. When writing, always look things up. Whenever you're not sure how to use a word, look it up in a good dictionary to find example sentences with it. When you've written something, and you are not sure if it's correct, search for it on the Web with Google. If many pages contain your phrase, then it is probably correct. Dictionaries and Google should be your everyday tools, and you should use them even many times in one sentence (especially if you've just begun writing in English). See this forum topic for more information on using Google when writing.
5. Know where you can screw up. Sometimes learners don't even realize how different English is from their native language. When speaking, they translate word for word from their native language, and they think their sentences are okay.
   When reading or listening in English, pay close attention to things like word order, articles, prepositions, and tenses. Compare sentences in English with equivalent sentences in your native language. Notice the differences in words and in word order. This will make you more careful when speaking in the foreign language, because you will realize which parts of your sentences can be wrong and should be double-checked. By Hassan Ali

"Will I ever be fluent if I speak so slowly and carefully?"
Don't worry about fluency. Fluency is easy to achieve by simply talking. If you practice speaking, you will be able to speak faster and faster. In high school, Tom achieved pretty good fluency in a month by talking to his English teacher (a native speaker) about two hours a week. That's only 8 hours of talking.
In our opinion, it is much better to be slow and correct than be fluent and make a lot of mistakes. Why? Because if you are slow and correct, you can easily improve your speed and become fluent and correct. But if you are fluent and make a lot of mistakes, it is not so easy to remove your mistakes and become fluent and correct By Hassan Ali


Don't speak or write too early

If you follow the above rules and still make a lot of mistakes when speaking (= more than 1 mistake every 3 sentences), you should probably switch to writing for a while. Consider the following guidelines:

1. First write, then speak. Writing is easier than speaking because: 1) you don't need to have good pronunciation (but you need good spelling), 2) you can write very slowly and nobody will mind, 3) you can use dictionaries, the Web, etc. So, it's a good idea to practice writing first until you can build correct sentences quickly enough for speaking.
2. Don't speak until you've learned to pronounce English sounds. You need to be able to pronounce all the English vowels and consonants in a clear way before you speak. If you don't, you will get used to bad pronunciation.
3. Don't say a word if you don't know how to pronounce it. In other words, you need to know the pronunciation of all the words that you use. If you don't, you will be making pronunciation mistakes and teaching yourself bad habits.

If you make more than 1 mistake every 3 sentences when writing in the slow and careful way described above, you should probably stop writing for some time and concentrate on reading and listening.
Remember that you should first get lots of English sentences into your head, then build your own sentences. Your main activity should be reading and listening to English - and the reason is that you need good examples to follow before you can build your own sentences. The more sentences your brain absorbs, the more you can express in English. If you don't see/hear enough correct, natural English sentences, you will not know how to say things in English. So you will be inventing your own language. And you will be making mistakes.