What comes to mind when you hear the word "teacher"? Imagine that someone who has a professional in a certain field, who can transfer its own experiences to somebody something under any condition and who is very patient and knows how to bond with the student. This should not be limited only in the field of English language teaching, how should a teacher be when you look at it comprehensively? Anyone who wants to teach, no matter what field he or she is in, must have some knowledge in this field.
Some of us apply to some programs for this subject, such as TESOL / TEFL or CELTA / DELTA. What could be the reason for you to resort to this? A person knows the English language but he/she has to learn how to teach the language. There are teaching techniques and approaches we need to learn. The teacher will develop her/his techniques by learning techniques which are given by these courses. As well as every student, someone who wants to teach in this field should take a guiding course. The teacher must find her/himself in this didactic area.
If someone asks about the main point of this statement, not every person who knows English can teach, but anyone trained in the field of English teaching can teach English. It's not just about knowing. It means to endear students to the lesson, to make it fun for them, and to spend it for them. Teaching is not just about explaining any topic. It is knowing how to convey this.