"This plant is smaller than this tree."
Translation:यह पौधा इस पेड़ से छोटा है।
इस is the oblique case form of यह.
If you don't know what the oblique case is, it is whenever there is postposition following a clause or word.
For example, खाना changes to खाने when followed by a postposition like से or में. In this sentence "यह पेड़" changes to " इस पेड़ " because it is followed by the postposition, से.
By the way, पेड़ does not change with the oblique case because it doesn't end with an "aa" sound. खाना changes with the oblique case because it does end with an "aa" sound.