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I think it's correct to say
"Recently I have been feeling ill" - feeling ill from the past to the present.

But I wonder if I can say " Recently I felt ill " - I am now feeling fine, but recently I felt ill.

Also, in the following examples

" Several recent studies have used the new technologies to show that ... "

" Several recent studies used the new technologies to show that ... "

" Several recent studies have shown with the new technologies that ... "

" Several recent studies showed with the new technologies that ... "

Are all four the same?

Thanks
Comments  
SeraphinI think it's correct to say
"Recently I have been feeling ill" - feeling ill from the past to the present.

But I wonder if I can say " Recently I felt ill " - I am now feeling fine, but recently I felt ill. Yes

Also, in the following examples

" Several recent studies have used the new technologies to show that ... " Yes

" Several recent studies used the new technologies to show that ... " Yes

" Several recent studies have shown with the new technologies that ... " Yes

" Several recent studies showed with the new technologies that ... " Yes

Are all four the same? Yes, but it depends on context

Thanks

Is it correct to say, with regard to the context, 
for simple past - the sentence "recent studies used .. " means they don't use the new technologies anymore or the sentence simply wants to emphasize the action of the verb "to use" that those studies "used" those technologies. 

I have problems interpreting the usage of the verb "to show".

Thanks
Students: We have free audio pronunciation exercises.
Seraphinfor simple past - the sentence "recent studies used .. " means they don't use the new technologies anymore
That sentence doesn't tell us anything about what may or may not be in current use.
SeraphinI have problems interpreting the usage of the verb "to show".
Demonstrate or prove would be synonyms in this context.