Q:

Combine vs join vs unite vs link differences in meaning

Explain, please, in detail exact differences in meaning between:
1 to combine
2. to join
3. to unite
4. to link

A:

All of these words can be used in many overlapping contexts, often as synonyms.
To "combine" two separate things is to put them together into a single grouping, mixture, etc. This group is then referred to as a "combination" of the individual elements.
To "unite" also implies grouping together separate things, but conveys a stronger sense of "bringing disconnected things together", or "merging many things into a single unit".
To "join" things also means to bring them together into a grouping, but generally conveys a sense of the two things melding together. To join implies "becoming a part of" or "becoming attached to". When a person joins a group, they become a part of it. When a person joins two pieces of wood, they physically attach them together ... etc.
To "link" two things is to form a connection between them, and can often be synonymous with "join". However, "link" doesn't convey as much of a sense of the two things becoming a unit, but more that they are connected or related. Thus we might say that "there is a link between smoking and cancer", because there is an important relationship between them. "Linking" also conveys a sense of a "chain" of interlocking connections (and the etymological origins of the word reflect this). The links of a chain, or links on the internet, are both examples of this latter sense.

