sentence #1: the clause "thinking ..." is too distant from the word that it modifies ("fellows").
sentence #2: There is nothing wrong with this sentence but since you tend to overuse present participles, I suggest "way to learn" instead of "learning method".
sentence #3: The only verb in this sentence is "is", which does not harm anything but neither does it contribute anything to the sentence. It is better to find a verb that has a significant meaning.
sentence #4: This sentence is hard to understand. It took me a while to figure out that you were talking about your native language rather than English. This sentence relies too heavily on present participles. There is nothing wrong with present participles, but it is possible to use too many of them. Also, present participles have a habit of displacing real first-class verbs. The verb in your sentence is "to spend". However, the most important ideas in this sentence are "speaking" and "learning", not "spending". Thus I suggest you use "to speak" or "to learn" as your main verb.
sentence #5: Your only verb is "to be". Instead, use a verb that contributes to your thesis. The main idea in this sentence is contained in the word "difficult", so I used a related word ("challenge") for a verb.
sentence #6: This sentence doesn't make sense. I am especially confused by "turns into".
I'm not sure whom you mean by "fellows".