"The fragrance of this plant is good."
Translation:इस पौधे की ख़ुशबू अच्छी है।
This is because of the oblique case. There are various postpositions in Hindi that trigger this case. Here, the "की" tells "यह पौधा" to put itself in its oblique form, so "यह पौधा" turns into "इस पौधे."
Nouns also get slightly tweaked in the oblique case.
Here's a helpful article about when to use the oblique case: http://www.learning-hindi.com/post/1133921729/lesson-50-postpositions
And here's another one about how to get a noun in its oblique form: http://www.learning-hindi.com/post/1116750602/lesson-48-nouns-in-the-oblique-case