Hello Samuel, The past perfect tense is used to talk about events/actions before a particular time in the past for example: 'I had read the whole book before he arrived', or ' By the age of five Bobby had learnt to read and write'. Present perfect is used to talk about past events together with their present results - it links the past with the present, for example: ' I have lived in Scotland for 10 years. I still live there now' or 'I have dropped the glass. There are still bits of glass on the floor'.

Past perfect is used in a sentence to describe an action that took place before some other past action. "I had finished my chores before father came home." Present perfect is used to describe something that happened at an unspecified time before now. "No one has ever solved that mystery."

这么说吧，现在完成时要记住三个要点：发生在过去，持续到现在，对现在有影响，第三点尤其重要（这是它与过去时的一个区别），这个法则适合绝大多数情况。比如说，i have gone to US,说明你已经去了，并且没有回来，现在还在美国。当然，也有一些例外，比如i have finished my homework,并不是你现在还在写作业，这只是表示一个完成的状态 过去完成时简单的说就是用在过去的完成时，一般要看见相关的句式和标志性词语才会用。一般来说会有一个过去的时间点来做参照，这之前的就可以用过去完成时。比如说I had finished my homework before i arrived the US. 很明显你到美国是在过去， 你在那个过去的时间点之前就做完了作业。

Yes, Samuel, MoiraWendy is right. Study your book. Do some work yourself. I have seen your other questions. You often forget to show what work you have done, such as looking up vocabulary in a dictionary, and which explanations or interpretations you find difficult to understand, and why.

Your grammar books should explain the difference. They should also have timeline diagrams and example sentences. Study your book.... then ask more specific questions if you are still confused. It is a huge area of grammar.