What matters vs. what matter when starting a sentence

relative-pronouns grammar phrases

When you say "what matters most" you're either talking about a single thing: the thing that comes top of the "mattering" charts, if you like, or a group of things which are all at the "top of the mattering charts".

Therefore, use the singular form, ie "What matters most is <a single thing>" or the plural "What matters most are <a group of things>"

"What matter most" should never be used, although you could say "The things that matter most are ..."

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