Tämä sääntö koskee kaikkia.

This rule applies to everyone.

Breakdown of Tämä sääntö koskee kaikkia.

• tämä: this
• sääntö: the rule
• kaikki: everyone
• koskea: to concern; apply to

Questions & Answers about Tämä sääntö koskee kaikkia.

What does each word in Tämä sääntö koskee kaikkia do grammatically?

• tämä = this. Here it modifies sääntö, so it works like a determiner: this rule.
• sääntö = rule. This is the subject of the sentence.
• koskee = concerns / applies to. This is the 3rd person singular present form of koskea.
• kaikkia = everyone / all people / all of them, depending on context. Grammatically, it is in the partitive plural.

So the structure is basically:

[This rule] [applies to] [everyone].

Why is kaikkia in the partitive, not kaikki?

Because the verb koskea in the meaning concern / apply to normally takes its object in the partitive.

So:

• Tämä sääntö koskee kaikkia. = This rule applies to everyone.
• Asia koskee minua. = The matter concerns me.
• Se koskee meitä kaikkia. = It concerns all of us.

This is something English speakers usually just have to learn together with the verb:

• koskea + partitive = to concern, apply to

So it is not random; it is part of the verb’s grammar.

What case are tämä and sääntö in?