OK, my friend, now, forget what Carol Chen and i have said before, because these are not correct. let me tell u the truth. this is a little complex.
when we describe a state of something without any emotion, we use this structure:
subject + be + adj -----structure a
when we describe a state of something with some emotion, we use this structure:
subject + verb(not be) + adv ------ structure b
for example,
u r tall.
你是高的。-----structure a
你长得高。-----structure b
(长 means grow, 得 must be used for the structure "verb + adv")
when we talk, we almost always use the structure b.
for example, when u meet the child of your friend, u find that he is tall, then u gonna say: u r tall.
but for the same case, chinese gonna say: 你长得高.--structure b (they never ever say 你是高的)
"u r tall" is a state, right? u dont want to express any emotion, right? why do chinese use the structure b, instead of a?
i think we could say this is the difference of culture.
in china, when we say: u r tall. it means that u are taller than what i thought. so here, we want to express a kind of emition: ur size shocked me. so we use the structure b.
what is "to describe a state without any emotion" for chinese? in other words, when do chinese use the structure a?
look at this: tree is tall. flower is short.
for example, when u teach a little baby english, maybe u gonna say: tree is tall, flower is short. in this case, chinese gonna say: 树是高的。花是矮的。-------structure a
but when u see a tree and u want to tell ur friend that this tree is tall by chinese, u must say: 这棵树长得高。-----structure b it means that u want to express that u are shocked by its size.