common grammar mistakes

I made many grammatical mistakes throughout this course. I am happy that this course has taught me a lot of ways to improve my English.
A confusion that I have is the usage of “had” versus “have”. For example, the sentence is “The school has a canteen on level 2”. I have always reminded myself that it is not appropriate to use had in this sentence as the canteen is still at level 2. I believe why this problem arises is because in the past, I was taught to use the same tense for my composition. Thus, I have the tendency to use past tense for my composition.
Another mistake that I commonly make is to put speech into words directly. Until recently, I finally realize that it is not appropriate to do that as it sounds very informal. For example, I wrote the phase “pros and cons” in my report. I was told that it is inadvisable to do so and doing it makes the report informal. There are many better ways to express the idea.
The third mistake is I sometimes have trouble identifying whether to use which, whom, who and where. For example, in the past, I was told that the word “where” is associated with a place. However, I do not understand where people can use “where” to associate with time. For example, “There will be months where there are only four Mondays as the first trading day of the month is not a Monday”. It is very confusing as my school teacher always told me to use when for something that is associated with time.
These are three common mistakes that I make. I believe that I have made many others along my journey of learning English language. In few years time, I hope to master the language so as to write a perfect report!