What are the most important differences between the American President and the British Prime Minister?

There are a few important differences we need to understand between the President and the Prime Minister. These are probably the two most important and if you remember them you will be doing well.
Firstly the President is the head of state and holds all executive power and therefore can act alone even when others disagree with him. The Prime Minister is not the head of state, the Queen is the head of state and the Prime Minister does not hold all executive power but rather shares it with his cabinet of senior ministers. This means that all executive or government decisions have to be decided and agreed by the whole of cabinet not just by the Prime Minister - he cannot act alone. Secondly, the president is directly elected and is totally separated from the legislature (Congress), he does not attend or have direct influence over Congress or over the legislation it puts in place. On the other hand the Prime Minister is simply the leader of the biggest party within the House of Commons. Therefore unlike the president he sits within the legislature and therefore has a direct influence over the legislation it puts in place.