Learn Finnish: Älä peitä oikeaa linssiä. - Do not cover the right lens.

In Finnish, negative sentences almost always require their object to be in the partitive case. Because you are giving a negative command (Älä peitä), the object must be in the partitive form oikeaa linssiä.

What is the base form of the verb peitä and why does it lose a letter t?

The base verb is peittää (to cover). When making a singular command, you use the weak stem. This triggers consonant gradation, weakening the strong -tt- into a single -t-, which gives us peitä.

How exactly do we build negative commands like this?

For a singular negative command, use the negative imperative word älä (do not) followed by the exact same verb stem that you would use for a positive command. So peitä (cover!) simply becomes älä peitä (do not cover!).

How would the object change if this were a positive command?

If you wanted to tell someone to cover the right lens, the sentence would be Peitä oikea linssi. In a positive singular command, a total object takes the basic nominative form instead of the partitive.