Hi 
With this topic , i have some opinions .
Firtly , in the sentence : "Nature is [color=red]a best teacher"as this phrase [color=red]suggets, one will always need a medium to learn new things and new ideas.Personally I belive that one can learn better with a teacher [color=red]rather [color=red]learning alone by oneself . I think that i will write “nature is[color=red] the best teacher ( replace “a” by"the” because “best” is a superlative comparison ) is as this phrase [color=red]suggestion , one will always need a medium to learn new things and new ideas .Personally, I belive that one can learn better with a teacher than [color=red]by himself (as i know , in toefl essay , the writer had better write as shortly as possible but concisely and clearly )
In sencondly phrase , "alone"in “when you are studying alone” should be replaced by “yourseft” because “alone” shows a condition . It means that someone haven’t got married , still single . And also , if i were you , i would use simple present instead of present continuous because this sentence expresses a truth which is your idea .
“you will have [color=red]lot of questions [color=red]to be answered” should be writen "you will have [color=red]lots of questions [color=red]which need answering "
“also , if you solve a problem and stuck [color=red]by some the teacher can be the best guide [color=red]to give answer to question "
" teacher is [color=red]consider as [color=red]a God” can be writen “teacher is [color=red]considered as [color=red]the God” because this is a passive voice structure . And then , also , to avoid using the same word , you can write “because [color=red]they are …”
“Then we went our professor and he gave us some clues to solve it and the reference of the another book so you can imagine if we had to solve the problem alone it could have taken ages” . In this sentence , i can understand your meaning but truly , it’s not really clearly and a bit complicating . so sorry for my idea :? and also , forgive me , hah ??
. Hmmm , i write :“Then , when we met our professor , he gave us some [color=red]ideas (better than [color=red]clues , i think ) to solve it and some books of reference . Through my incident , i believe that you can image without experience and suggestions of my teacher , how complicated we were .”
In last sentence , my suggestion is "In conclusion , to [color=red]achive knowledge more accurately andthoroughly , teacher is a good medium ( i write so because “medium” is a thing which has responsibility to link two objects . So , it’s better for you to show two objects there . these are : people who study and knowledge which they need archive )
And another opinion , i really like your conclusion . You are right . the teacher always "guide us in the dark and protect us [color=red]from ( :arrow: IDM : protect sbd/sth from sbd/sth ) harsh sun. "
Hmm…at last , i think that you can give some more phrase to show how necessary the teacher is for our life . For example : without them , we don’t know which position we are in to continue trying and training , we don’t know our false and our defects … there are only them who can show all of them for us . and also , you can quote a lot of famous saying to prove your opinion right
i hope my above opinions useful for you . wish you luck ! 