The usage of 'have' and 'had'

Lyangsuan wrote:

When do we use "had" instead of "have"? Is the verb behind "had" a past tense or present tense? Example. Teenager who had finished their secondary education. Teenager who have finished their secondary education. Teenager who had finish their secondary education. Which one is correct?

"have/has" is present tense:  I have a headache.  "had" is past tense:  I had a headache last night.

BUT, your question here is about compound tenses, using the helping verb + the past participle of the main verb.  Present perfect with 'has/have' refers to an action completed before 'now', the present.  Past perfect with 'had' refers to an action completed before another past action.

I have eaten there many times [before now].

He had eaten all the cake before the guests arrived.

Using this, see if you can 'clean up' your suggested sentences with more information.

Hmm let me try then.          

Teenagers who had finished their secondary education. --this sentence is past perfect. It doesn't mean anything much without a context. It would be okay if you said teenagers who had finished their secondary school were accepted  to the new high school of our city.
Teenagers who have finished their secondary education. --this sentence means teenagers are secondary school graduates.
Teenager who had finished their secondary education.--- this sentence means they finished their secondary education in the past.   

You can't use had finish. Either do your sentence in past tense and say teenagers finished ... or do it in past perfect tense and say teenagers  had finished...

Look in the past/present perfect tense you use the V_3. It doesn't have to take -ed. Just choose the third type of the verb e.g.     

V₁                      V₂                   V₃

See              Saw              Seen

Come           Came          Come

Cut               Cut              Cut

Get               Got              Gotten   

Drink            Drank          Drunk

 

Anonymous

Hi,

I couldn't get it properly. You mean this sentence: 'I have had a headache'  is not present perfect tense?S

No, it is NOT present tense.

I have a headache is present tense. I had a headache is past tense.
I have had a headache for the last hour is present perfect. I had had a headache before the concert and it only made it worse is past perfect.

It's harder to understand with "to have" because we use "to have" for form the perfect tenses as well as using it as the main verb. It's easier to see with other verbs.

I see a deer right now. I saw a deer in the neighbor's yard. - Present and past, respectively.
I have seen many deer on this road at night so drive slowly. - Present perfect
I had seen the deer, but I thought it was safely off the road, and didn't expect it to jump back in front of my car. (Good thing I have good brakes!) - Past pefect