There are two important aspects of context:
1. Where did the text come from? Your sentences may have come from a textbook, a blog, a poem, a song lyric, an action novel, an email, a diary, a love letter, or a conversation. This is often important, because a sentence might work just fine if it is spoken between friends, but it wouldn’t be appropriate in, say, an email to a boss. So it’s not fair to us to ask: “What does this mean?” or “Is this grammatical?” or “Which of these is better?” when we have no idea where it’s coming from or what it might be used for.
2. What is the surrounding context? Many times, sentences can’t be analyzed on their own. We need to know what the surrounding context is. (This gets discussed with an example at our meta question called Why you should cite your source.)
With that in mind, you asked:
So if you are writing a letter and you are confused between two similar sentences, how will you give the link that where you found the sentence and other context?
It’s quite simple: If it’s something that randomly came into your mind, tell us so; if you are writing a letter, you can simply tell us you are writing a letter. (In the case of a letter, it might also help to tell us the nature of your relationship, too – are you writing a penpal, or are you writing your doctor?) So, you might start by saying something like this:
I am writing a letter to my life insurance company, because they have asked me to tell them about my exercise regimen. I’m confused about which of these two sounds better:
- A: I exercise daily
- B: I do exercise daily
Now we have a better idea of what you are trying to say, so we have enough information to give you a proper answer. Otherwise, writing an answer can be very complicated, because we are wondering: Why are we being asked this question? How does this person intend to use this sentence?
The Stack Exchange can be rather frustrating when you spend a good amount of time writing what you think will be a helpful answer, and then, shortly after it’s posted, the OP leaves a comment that not only better explains the original question, but also pretty much nullifies everything you wrote in your answer.
On the other hand, if this was something that “randomly came into your mind,” your question might look a little different:
I was thinking about writing sentences in English, and I noticed that these two sentence structures seem to say the same thing:
Is one of these better? Is there any subtle difference between them?
(Notice how I made my second example a little different from your second example. By asking about exercise and running, it makes it seems like your question might apply only to strenuous physical activity. By changing that to exercise and painting, the emphasis shifts away from that particular activity and focuses more on the sentence structure.)
By the way, one other tip: Try giving your question a title that is more meaningful. Assume someone remembers your question six months from now, and they want to search for it. A title like, “What is the difference between these two sentences?” might be used in dozens of questions. (Egads! What a mess.) A title like, “Difference between ‘I exercise’ and ‘I do exercise’” would be much better.