Learn Finnish: Kastele kaikki pensaat tänään!

This sentence has a plural total object. Because the command is to water the whole group of bushes completely, the object is total. In Finnish, a plural total object uses the nominative plural form, which ends in -t.

How do we get the plural form pensaat from the basic word pensas (bush)?

For nouns ending in -as or -äs, the stem changes by dropping the -s and lengthening the vowel. So the stem of pensas becomes pensaa-. Then, you simply add the plural marker -t to get pensaat.

Why doesn't the word kaikki get a plural -t ending to match pensaat?

The word kaikki is a bit of an exception. When it means "all" and modifies a noun in the nominative plural form, it stays in its basic singular form. We say kaikki pensaat, never kaiket pensaat.

What is the base form of the verb kastele, and how is this command formed?