Necessary means required or essential, while needed means required or essential for a particular purpose. While both words imply that something is required, necessary is more absolute in its meaning. It suggests that something is vital and cannot be done without, whereas needed implies that something is required for a specific purpose or situation.
from English Grammar Today. We use if to introduce a possible or unreal situation or condition. We use when to refer to the time of a future situation or condition that we are certain of: You can only go in if you've got your ticket. When I'm older, I'd love to be a dancer. Compare.
If you say that something will happen if necessary, when necessary, or where necessary, you mean that it will happen if it is necessary, when it is necessary, or where it is necessary. If necessary, the airship can stay up there for days to keep out of danger.
At the start of a sentence, use a comma after "if necessary", "if needed", "when required", "where needed", etc. Normally, we don't need a comma before "if necessary", "when needed", etc. in mid-sentence or at the end of the sentence.
Opinion Columnist. On Sunday, CNN's Dana Bash asked Representative Pramila Jayapal why so many progressive women have been silent about the extensive reports of widespread rape and sexual
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if necessary or when necessary
