那么, 如此 and 太, these words are functional as adverb, it modify the word 难. Your chinese writting in these three sentences are grammatically correct and understandable. Which is better? It depends on you, the wirtter, I mean the person who put it into application. In English, it is called the writting style. Many people who claimed themself Native Chinese speakers or scholars with their attempt to confuse the learners by giving lots of definitions. In my opinion, all sentences mean the same, it expressed the idea of leaning Chinese is very difficult.

Yes,you can say 我觉得学中文那么难！ 我觉得学中文如此难！我觉得学中文太难了！ But 我觉得学中文太难了！is the best. I this phrase,那么and 如此can used "so",太can used "too" 那么, 如此 and 太,they are almost the same in most times.

Hi,伊丽莎白。In my feeling,When you use them to expression levels,they have almost the some meaning.But in the other way the word “那么” can be expressed as a turning point,the word"如此"and“太"do not.

in my opinion ，it just like translating the word "so" in English sentences to Chinses .sometimes when i reading, i don't know exactly it should translated to 那么、这么、太…… but i can feel it's meaning. So 伊丽莎白 ，i think you say :我觉得学中文那么难！ 我觉得学中文如此难！我觉得学中文太难了！that's all right ,all make sense ,there is no mistake in grammar.

all the three is ok.but you'd better say"中文那么难。中文如此难。我觉得学中文太难了。”and the last one is the most like the original tougue.