1. Subject verb agreement
    “They is going” is wrong; “they are going” is right, and that seems obvious.
    If the subject is plural, the verb has to be too.

  2. Use of whom
    When who is the subject it should be whom. Your friend tells you of some colourful company and, surprised, you say “you went to the party with whom?”.

  3. Then and than
    Than is used when you’re talking about comparisons e.g. “I outran the hungry tiger easily; I am faster than that”.
    Then is used when you’re talking about something relating to time e.g. “I left the party earlier then you and headed home for a cocoa and early night”.

  4. Use of less or fewer
    Less should be used for something that is singular or intangible e.g. “It was widely acknowledged that Donald J. Thump possessed less of a moral compass than his predecessor”.
    Fewer is used in the case of a discrete multitude, as in the example of the express lane at the supermarket being for “Ten items or fewer”.