Three common grammar mistakes

I have learned English as my second languge for about eight years. During the course of study, I find I often make grammar mistakes. Among all the mistakes I usually make, the three most common ones are shown as follows.

1. Article
Different from Chinese, English usually have articles within a sentence. An article is a word that combines with a noun to indicate the type of reference being made by the noun, and may also specify the volume or numerical scope of that reference (Wikipedia). The articles in the English language are the and a(n). Different articles should be used for different situations.
e.g. mistakes: NUS graduates are well esteemed by the employers at the workplace.
Correction: NUS graduates are well esteemed by employers at the workplace.
Reason: NO article is used when a plural contable noun is nonspecific.

2. Transition
Transition words allow one to work smooth changes into one's writing; thus, simple sentences turn into compound sentences, complex sentences and long paragraphs. Transition words provide a powerful tool to put sentences and phrases together, bringing coherence and consistency, and, most importantly, they put the thoughts one's mind is trying to convey into written words.
e.g. mistakes: Although academic studies are challenging, but most students will persist in their efforts.
Correction: Although academic studies are challenging, most students will persist in their efforts.
Reason: although should not be followed by but.
Also, hence, therefore, thus should be followed by a comma. There are commas before and after however if it is used within a sentence.

3. Modals
A modal verb (also modal, modal auxiliary verb, modal auxiliary) is a type of auxiliary verb that is used to indicate modality. Common modal verbs are: can, could, may, might, should, must. Different modals are used to represent different possiblity and necessity.
e.g. mistakes: If the global warming is not mitigated soon, global disater should happen.
Correction: If the global warming is not mitigated soon, global disater could happen.
Reason: This is just a assumption. Could is used to represent the possibility.

Grammar is something interesting and sensitive. A slight grammar error may lead to completely different meaning. Hence, I will try hard to master it and use it in a correct way.