grammar problems

[lisa***]

hi good day everyone!

i'v been teaching grammar for koreans for some time. however, i am often faced with difficulties of explaining things i didn't even think about. please help me with some of these difficulties such as:

1. which is grammmatically correct?
she has beautiful wavy short hair . or she has beautiful short wavy hair. ( what category does the adjective wavy belong to? --shape or condition?

2. instead of saying " i've drunk four cups of coffee in an hour." my student thinks it is also possible to say...
i've been drinking four cups of coffee for an hour.
( my student intends to emphasize the question " how long?"
and how many cups at the same time) does the sentence make sense and is it grammatically correct?

3. lastly, we ask the question " what do you think will happen?" i felt really stupid when i couldn't explain why it is not possible to say...
what do you think it will happen ?
please enlighten me.

thank you so much and more power.

[Casiopea]

1. She has beautiful short wavy brown hair.

observation: beautiful
size: short
shape: wavy
color: brown

2. I've been drinking four cups of coffee for an hour.
=> it means, you've been taking sips from four different cups of coffee for the past hour.

3. What do you think it will happen?
=> 'What' is the subject of the verb 'will happen'. There should be only one subject per verb:

You think what will happen?
What will happen do you think?
What do you think will happen?

Hopes that helps.