Present Perfect tense
We use "have or has" (present tense) with the past participle (past tense) of the second (main) verb to form the present perfect tense.
e.g., We have known for years that smoking causes lung cancer.
e.g., I have travelled to China twice in my life.
e.g., I have asked a good quesiton.
We use this tense to talk about things in the past which continue to be true.
e.g., She has lost her keys.
This means that her keys are still lost, or remain lost. If you use the simple past tense and say that "She lost her keys." it means that she lost the keys in the past, but may have found them again. If you tell someone "she lost her keys', they are likely to ask 'Has she found them yet?'.
But if something (an action or event) started in the past and has continued up to this time we can say:
e.g., We have studied English for many years (and are still studying).