1. The grass grows thick in the garden.

2. The grass grows thickly in the garden.

3. The grass grows thin in the garden.
4. The grass grows thinly in the garden.
(Which expressions are correct and grammatical?)

Since I could say, "The grass is thick [or thin] in the garden," and keep the same meaning, I would use sentences 1 and 3.

http://www.chompchomp.com/terms/linkingverb.htm
Scroll down a bit and read all about the "multiple personalities" verbs!!

Both expressions are grammatically correct, but they convey slightly different meanings. Let's break it down:

1. "The grass grows thick in the garden."
This sentence implies that the grass is thickening or becoming dense in the garden. It focuses on the physical quality of the grass being thick.

2. "The grass grows thickly in the garden."
This sentence indicates that the grass is growing in a thick manner in the garden. It emphasizes the manner or way in which the grass is growing.

Now let's move on to the sentences about thinness:

3. "The grass grows thin in the garden."
This sentence suggests that the grass is thinning or becoming less dense in the garden. It is highlighting the physical quality of the grass being thin.

4. "The grass grows thinly in the garden."
This sentence describes how the grass is growing in a thin manner in the garden. It emphasizes the manner or way in which the grass is growing.

In summary, all four expressions are correct grammatically, but the choice between "thick" and "thickly," as well as "thin" and "thinly," depends on whether you want to emphasize the physical quality or the manner of growth.