Student or Learner
It is very difficult for me to know well when the prefix dis, un and im be used. Is there any way to memorize or anything else, by which I do not make mistake in making sentences. As all are negative. Thank in advance for the answer.
Unfortunately, there is no easy way of knowing which prefix any adjective will use to form its opposite. When you learn a new adjective, note down whether it has an opposite formed with a prefix.
in- becomes im- before a root beginning with "m" or "p":
e.g. immature, impatient, improbable, etc. Similarly in- becomes ir- before a word beginning with "r", and il- before a word beginning with "l":
e.g irreversible, illegal, illiterate.
Although it is mainly adjectives which are made negative by prefixes, un- and dis- can also form the opposites of verbs too, e.g. appear- disappear. The prefix is used here to reverse the action of the verb. Here are some more examples: disagree, disapprove, disconnect, dismount, unbend, undo, undress, unlock, unwrap, unzip, etc.
You can use a good dictionary which can be of real help when talking about prefixes : Visual Thesaurus, version 3 or Word Web Pro, or any other dictionary.