Tenses are about time. A native speaker of English can usually use most tenses accurately, without knowing the names of them. They are more problematic when people learn English, or another language, as an additional language.
There are many different tenses which show slight differences in time but these are the most common ones.
I teach in Robertson High School.
Used for actions that happen regularly (even if they are not happening right now) and for things that are generally true.
Present continuousI am teaching.
She is teaching Shakespeare just now.
Used for actions that are going on now but which may be temporary.
Past simpleI taught.
She taught until she retired.
Used for actions in the past which are finished.
I have taught.
Used for actions which started in the past which are still going on or which were completed in the past but are still directly relevant in the present.
(Note this is a past tense although it contains the word 'present'.)
I had taught.
Used for actions which happened before an event that is already in the past. (This tense used to be called the pluperfect.)
Future simpleI will teach.
She will teach Shakespeare next period.
Used for things that will happen in the future – even if it is just a few minutes in the future.