Make yourself a "block" of text, some short lines, some long.
Ensure there is a blank line after the block of text.
Do a block copy (or cn, where n=lines including blank) including the blank line, or even better a block repeat including blank line, or if you prefer type out two new paragraphs.
Save the file.
At the start of the first block, in the prefix area, put TF at the top of the block. In the second block, put it somewhere in the middle. In the third block, put it on the last line of text. Press enter. Note what happens. Report back here with results.
Cancel the change. Edit the file again. Remove the first blank line. Put TF on the first line of text (in the prefix area). Press enter. Note what happens. Report back here with results. Cancel the change.
Carry out further experiments under independent control. Look at what NicC has suggested. Look in the Tutorial, PF1, find out what affects how TF operates (it will tell you, better than I can). Experiment until you understand everything, or if you get stuck, come back and tell us what you have found and what you have a problem with.