- "Happiness can be found, even in the darkest of times, if one only remembers to turn on the light." 2. "You think the dead we loved ever truly leave us? You think that we don’t recall them more ...
The 'one hand' could be any generic hand (no article) or a specific one (definite article). 'The other' refers to the 'matching hand' of the first one. There can only ever be one, and it is being referred to explicitly as the partner of the first, therefore it needs a definite article.
Both 'a single' and 'one single' are correct and commonly used phrases in English. They are interchangeable and can be used to emphasize the singularity of an item or person.
The difference is that "one" puts more emphasis on the number such as I have 4 computers and a printer/I have 4 computers and only one printer. So when you say a feature of my work or one feature of my work, it means the same, with the only difference that "one" is emphatic compared to "a".
The "one" could imply that of the alternates only ONE choice is possible, or permitted. "Which" alone could indicate several choices from the set of alterates could be selected in various combinations. Of course, speakers are often very imprecise about their meanings & intentions when saying "which" or "which one". Usually the context makes it clear if the choices are mutually exclusive or not ...
One-to-one is used when you talk about transfer or communications. You may use one-to-one when you can identify a source and a destination. For eg., a one-to-one email is one sent from a single person to another, i.e., no ccs or bccs. In maths, a one-to-one mapping maps one element of a set to a unique element in a target set. One-on-one is the correct adjective in your example. See Free ...