1. I thought it was a brand new song... after I've finished downloading (it ?), it struck me that it wasn't.
2. I thought it is a brand new song... after I've finished downloading (it ?), it struck me that it's not
a) is the part with "after I've finished..." allright? it the use of the pres. perfect tense apropriate here?
b) is it necessary to put "it" after downloading ?]
'I thought' is past tense, so you cannot follow with a verb in the present tense unless you are quoting your thoughts e.g.
I thought, "Hm, this is a brand new song."
'struck' is past tense. One of the uses of Past Perfect tense is to place one thing further back in time than another - you downloaded, then it struck you, so 'downloaded' is further back in time than when 'it struck'. You have used Present Perfect tense which is not correct.
"After I have downloaded the song, I will burn it to a CD."
Most people would understand the meaning if you omitted the first 'it' - it sounds very 'computer expert/geek talk'. Most native speakers would probably include it.
So - best sentence:
1. I thought it was a brand new song. After I'd finished downloading it, it struck me that it wasn't.
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