How do you have knowledge and understanding, of how to communicate effectively with others.

I could write a 'ton' of words here... but suffice to say;

LISTEN, ask a lot of questions, smile at appropriate times, stay concentrated, pay attention.

When you ask a question, respond to the answer with acknowledgment and another qualified question to their response/answer... show honest interest, always.

People do not always find a willing recipient who intently listens... you will build a rapport with this person and a bond.

Stringer

LISTEN! Listen not only to words spoken but also to the gist of their part of the conversation. Never interrupt with your rebuttals or answers but make a mental or written note. Allow the other person to have their entire say. This allows them to feel they have "gotten it off their chests" & will improve their respect for you. Most subordinates' complaints are about superiors that never listen to what they have to say.
Then it is your turn. Address each question & concern expressed by the other person. Once all concerns have been addressed then it is your turn to motivate the other person toward your line of thinking. TACT is allowing the other person to do it your way!
The words "I" and "you" should NEVER be used. The word "I" produces the feeling of Master over Slave. The word "you" points a finger at the individual and is generally perceived as a "put-down".
Substitute the word "we" for "I" and "you". Instead of "I want this cleaned up" say "we need this cleaned up". Instead of "you need to do something about this" try "we need to do something about this". This simple change makes it a team effort instead of a master over slave situation. This brings your team member into the team in their minds.
This works with bosses, employees, peers & spouses. The key word is LISTEN.

Know them by what they say, and do, paying close attention, and be a good listener. Listening carefully will enable you to learn much about a person.