Tenses

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Tenses, Present Perfect, Simple Past, Adverbs of Time
   adverbs of time  
present perfect tense.  
we’ve seen gases have increased present perfect tense
the level of greenhouse gases in the atmosphere has changed in just the last two hundred years.   present perfect tense.
to have    
tenses
simple , perfect and continuous .
we’ve found out
just
auxiliary verb
 ‘I have you have she has we have they have
changed found out seen increased helped
past participle

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Simple past tense      
There were much lower concentrations pre-industrially
pre-industrially before the industrial revolution’ in Europe.
simple past tense describes an action that occurred at ‘a specific time’ in the past.
present perfect tense describes an action that has been completed at ‘some indefinite time’ in the past.
 ‘just’            
last Christmas  on the 15th of May  in 1979  three million years ago
adverbs of time + present perfect tense
recently completed action ‘just’, ‘already’, ‘yet’ ,  ‘still

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‘yet’ ‘still’        
indefinite time ever     never            
 The level has changed in ‘just’ two hundred years.  The Earth’s atmosphere has ‘already’ been destroyed.
Have you finished your assignment yet?  No. I am still writing the introduction.
Have you finished your assignment yet?  No I haven’t written the introduction yet.
Have you ever been to China?  No, I’ve never been there.
a point in the past for’ ‘from’ and ‘since
I have been at this school for three months, since March.

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present perfect tense  statements about actions that began in the past and are still true now.
adverbs of time                
 simple past tense
 present perfect tense.
 adverbs of time