An い-adjective can be placed prior to something in order to modify it. 1
Example: The red pen is small
あかいペンが小さい
Info
あかい (red) is an い adjective that is preceding the noun ペン. It is applying the property specifically to the pen in this sentence, which means that it is a red pen.
あかいペン (red pen) is marked by が making it the core subject of the sentence.
小さい (small) is an い adjective that ends the sentence. This means that it is applying the attribute to the core of the sentence. The red pen is small.