No, they’re not ; sometimes they can be used interchangeably, but others may have slight differences in meaning or usage [nuances] which would make them unsuitable as substitutes.

Hey Hussayn, This is a great question. As Adrian has said above, no, synonyms are not always interchangeable. There are several reasons for this. One important reason is connotation. Some words have negative connotations and others might have positive ones, or a neutral one. An example of this would be "cheap" and "frugal." Both terms mean that a person doesn't like to spend money. But "cheap" has a negative connotation (it's an insult to say this about someone) while "frugal" has a more positive or at least neutral connotation. Another reason not all synonyms are interchangeable is formality. Depending on the context, you might want to use more informal or formal vocabulary. For example, firefighters can "extinguish" a fire. They can also "put out" a fire. The difference between these synonyms (extinguish and put out) is that the first is formal and the second informal. If you were writing for university, you would use "extinguish" while in normal life it would be "put out." One more thing I'll add is regional variation in the language. Depending on where you are, native speakers may prefer to use one synonym over another. You can see many examples of this. One is "revise" being used to mean "review" in the UK, in the context of reviewing homework, for example. I hope this helps! Make sure you learn connotations, collocations, formality and regional preferences when you learn a new word. It takes more time, but it is very worthwhile.

Good answers from Adrian and especially Eric From Canada. I'll add a third way that they're not interchangeable. Sometimes a synonym only matches ONE SENSE of a word. For example, "easy work" and "light work" are synonymous, but you can't ALWAYS substitute "easy" for "light". They overlap in the meaning of "not onerous or difficult", but they also have meanings independently.