帮助 vs. 帮
What is the difference between:
帮助 and 帮?
感觉 and 觉得?
Why isn't there 你的 between 注意 and 身 in 你要注意身体健康? How does one know whether you should take care your own body or someone else's?
觉得 is more like what do you think or how do you feel about something. What do you think about this dress. What do you think should be done. 感觉 is where we would use the word feelings or feel in that sense. Do you have feelings for her? How does it feel to be a mom?
Your last question doesn't have that because it's not referring to your body, just good health in general. If it was saying you should pay attention to your health it would have a 你的 in it.
你要注意身体健康= You should pay attention to your body's health.
As a native speaker, most of us would say 保重. You can choose to ignore the possessive in this case.
Native too. 保重(身体) sounds very polite and rather serious to me. I do not prefer this phrase if I often meet that person.
On (4): Similar default assumptions happen in English -- if I say "You need to shave", nobody is going to reply: "Who should I shave?"
(4) Context. “你的 (yours)” is the most likely.
(3) As a verb, they are mostly interchangeable. Both can be used to express the physical and emotional feelings, but 感觉 is more preferred when we focus more on sensation than on thinking. To place more weight on thoughts than on feelings we prefer 觉得 (weaker than 认为). But please note that this is just a false dichotomy for explaining the nuance. Also, we can say 去感觉它 (“Go feel it!”), no 去觉得它 (ungrammatical or unnatural).
For 帮助 and 帮, you'll find that this happens with a lot of verbs, actually. Many have single character versions and two form variants. Generally, the two character versions sound a bit more formal as their meaning is more specific, whereas the single character words may have a couple different meanings depending on context.