Number (n)

A problem with singular/plural is a number problem.  Many students forget to put an -s on plural nouns, or make mistakes with irregular plurals:

    1.  She is afraid of both snake and mouses.
         (should be "snakes" and "mice")

The more difficult problem is to know which nouns are count (and have a plural form) and which are non-count (and have no plural form):

    2.  She gave me a lot of advices. 
         (should be "advice")

Confusion about count/non-count can also create problems with articles and quantifiers:

    3.  She gave me an advice and many helps.
         (should be no article before "advice" and "a lot of" before
         "help" because both are non-count)

The links below give much more complete explanations, and exercises.

Links:

    Regular and irregular plurals:  plurals
     Count/non-count:  noncount
     More explanation and exercises:  noncount