Hi 

Can anyone answer my queries about the following sentences, please?

 

Hi, Ali.

1. Both are correct, but with different meanings. As a general statement, you need to use the first one. You would only use "I realized that I needed my computer" if you are talking about an event in the past in which you came to this realization. "I have realized that I needed my computer" is making a statement about your current situation (of being aware now that you needed your computer at some time in the past).

 

2. Both are correct, and they are nearly the same in meaning. In general, the continuous/progressive tenses are used to emphasize that an event takes place over a period of time, rather than just giving a piece of information. When you say, "Tom is currently working in Paris", the hearer can get a sense of the process. The simple present version is just a statement of fact.

 

3. I wouldn �t use "by"

 

4. "Science" is a general term, encompassing Chemistry, Physics, Biology, Zoology, Astronomy, etc. "Sciences" is not often used, and is usually used as "the sciences", mostly to emphasize that the term "science" refers to a number of disciplines. "Sciences" often has an adjective associated with it, too, such as "the physical sciences" or "the biological sciences."

 

Bruce 

Thank you great explanation