I have just had a cup of coffee.此句中有两个have(原形和过去分词)组成现在完成时态,这里have just had表示刚刚喝完(一杯咖啡).如果去掉just had,变成I have a cup of coffee.意思就变了,变成我有一杯咖啡(哈哈,楼主要不要喝呢?)