Learn Finnish: Opas varoittaa säästä. - The guide is warning about the weather.

The verb varoittaa (to warn) requires the thing you are warning about to be in the elative case (-sta or -stä). This fits the pattern of cognitive and communication verbs using the elative to express what the action is about.

If the guide were warning us, what case would the Finnish word for us take?

The person being warned goes in the partitive case. For example, to say that the guide is warning us, you would use the partitive word meitä to make the sentence Opas varoittaa meitä säästä.

Is varoittaa related to the verb varoa (to beware)?

Yes, they share the same root. The verb varoa means to watch out for something yourself. The -ttaa ending makes it a causative verb, meaning it causes someone else to watch out, which is exactly what it means to warn someone!

Does the subject opas undergo consonant gradation in other cases?

Yes, it undergoes inverse consonant gradation. While the basic nominative form has a single p, the stem for most other cases strengthens to a double p. For example, the genitive form is oppaan (the guide's).