Why do language learners make grammatical mistakes?
1st reason: language learners just don’t care.
2nd reason: language learners don’t read nor listen enough.
3rd reason: language learners don’t use monolingual learner’s dictionaries. No monolingual learner’s dictionaries, no grammatical accuracy.
4th reason: language learners don’t know how to study vocabulary and grammar, most language learners don’t have a method of learning vocabulary and grammar.
5th reason: Both, language learners and language teachers mistakenly believe that -- Understanding a grammatical rule or Understanding the meaning of a new word is enough to remember it and use it later on, use it quickly and correctly.
No, it’s not enough!
The widely held belief that: We should study to Understand, Not study to Memorise, doesn’t really apply to language learning.