When should place comma before the and in an English sentence?
I use Grammarly for checking my grammar, and it suggests me to add a comma each time I use and.
For example, "What you need to do is to create your website, somehow attract visitors, and place Google ads on your web pages."
Or such example: "I like blueberry, cherry, and grape."
I thought, should add a comma in front of "and" only when it connects two sentences.
I thought I should not add a comma when I list the items like in the second example, but Grammarly forces me to insert a comma. Is it correct for US English?

Question

When should place comma before the and in an English sentence?
I use Grammarly for checking my grammar, and it suggests me to add a comma each time I use and.
For example, "What you need to do is to create your website, somehow attract visitors, and place Google ads on your web pages." 
Or such example: "I like blueberry, cherry, and grape." 
I thought, should add a comma in front of "and" only when it connects two sentences.
I thought I should not add a comma when I list the items like in the second example, but Grammarly forces me to insert a comma. Is it correct for US English?

Answer

That is something called the serial, Princeton or Oxford comma. It's a stylistic option.
https://pwrites.princeton.edu/2016/01/28/defining-understanding-and-using-the-serial-comma/ 
Unless you are writing in a specific required style, you can use it or not, but it's probably a good idea to be consistent.

Answer

A comma is optional for the last item in a list preceded by 'and'.

In your example,  you could write  "I like blueberry, cherry, and grape." OR  "I like blueberry, cherry and grape." The important thing is to be consistent as Gary mentions. If you choose to use the comma, keep using it in following lists as well. And the reverse is true, also. If you omit the comma, do the same for the following lists.

Be careful with grammarly as well :)
It sometimes highlights possible errors where there are none. Just use it as a guide and nothing more.

Answer

thank you

Answer

As Gary and Greg have mentioned, the comma is optional. Also, if the list only has two items you don't need a comma. Ex: I like blueberries and cherries.

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