Learn Finnish: Orkidea on todella herkkä kasvi. - The orchid is a very sensitive plant.

When using the verb olla (to be) to describe what a singular, countable thing is, the describing words (known as the predicative) take the nominative case. Because an orkidea (orchid) is a distinct, countable object, herkkä kasvi is nominative. If the subject were an uncountable mass noun like coffee or water, you would use the partitive instead (for example, Kahvi on kuumaa).

Can I use tosi instead of todella in this sentence?

Yes, you could absolutely say tosi herkkä kasvi. Both words mean 'really' or 'very'. Tosi is extremely common in casual, spoken Finnish, while todella is slightly more standard and common in writing. Fun fact: todella is actually formed from the noun tosi (truth) put into the adessive case.

Is the noun kasvi (plant) related to the verb kasvaa (to grow)?

Yes! Kasvi is derived directly from the verb kasvaa. Adding an -i to a verb stem is a very common way to build new nouns in Finnish. In this case, it literally translates to 'a thing that grows'.