At first I thought the opening sentence of the introduction was the hardest to write. Then I thought that the body of the introduction was harder to write. But after giving the topic careful thought, I have come to a strong conclusion: The thesis is the hardest part of a paper to come up with.
The first sentence of a body paragraph is always fairly difficult. Sentences in the middle of a paragraph can be difficult, too. One time, I was writing a paper about trees, and I could not think of a specific example. The last sentence of a paragraph can be difficult as well.
Sometimes I can't think of more than one sentence to put in a paragraph.
Paragraph transitions are always hard for me. In addition, transitioning phrases can also be as well. However, punctuation is fairly easy; semicolons are especially great. Transitions at the end of a paragraph are problematic as well.
Another thing to consider is overly forced- or formulaic-sounding sentences. Another thing to consider is statistics. One study has shown that 85% of the time people have not been able to find appropriate statistics. Citing them could be a problem as well (Etal, et. al., 152). The last sentence of the last body paragraph is important, even though it only takes up 8% of the paper.
In conclusion, the thesis is the hardest part of the paper to write. The body of the conclusion can be difficult too, but remember: Figure out something to write for the last sentence.