That's quite a broad question. A good place to start to answer this is to consider both the personality and the approach to work of the teacher. For a teacher to effectively teach students, they need to fit with the level they are teaching, and they have to want to be there. The personality of a successful primary teacher will differ from that of an effective high-school teacher. High-school teachers tend to be more autonomous in allowing students a voice, whereas primary teachers are more likely to be significantly more controlling.
English learning success can also depend on how well a teacher sets up and maintains the classroom learning environment. The subsequent student behavior, and the attitudes that are created, impacts how well they will learn.
Lastly, the teacher must foster a positive and trusting relationship with their students. A classroom with a warm and open feel helps support the stability of the students emotionally, so they feel more connected to the teacher, which in turn leads them toward better focus, interaction, and aptitude.