When should I use "is" and when "are"?

Solution 1:

http://www.grammarphobia.com/blog/2006/12/all-is-vs-all-are.html

According to the link above :

“All” is a two-faced word. It can be either singular (“is”) or plural (“are”). If a writer means “all of it,” she should use “is.” If she means “all of them,” she should go with “are.” So it depends on whether your contributor was thinking of the whole dish or the various things in it: “All [the soup] is returned to a simmer” or “All [the ingredients] are returned to a simmer.”

Also in this forum: https://www.englishforums.com/English/AllIsOrAllAre/zdngg/post.htm

there was an interesting comment:

Meanwhile, I'll add that "all" is sometimes used to mean "the only thing". For example:

• All I want is you.

However, even if the "only thing" actually consists of more than one thing, we frequently use the singular form of the verb . For example:

• All I want for Christmas is my two front teeth.

I would also say that don't really check songs lyrics for grammar mistakes, because they sometimes use broken English in writing the lyrics!

Hope it helps you