A nearby meaning
A synonym is a word or expression with the same or nearly the same meaning as another in a particular use. Merriam-Webster’s definition allows for some senses or all senses. That small qualification matters when you choose a substitute.
Compare two short sentences: “I am happy to hear it” and “I am glad to hear it”. The words change, but the speaker still welcomes the news. Happy and glad work as synonyms in this example.
Let the sentence choose
Try replacing light in “a light room”. If the sentence means that the room has plenty of light, bright is a useful nearby word.
Now try “a light bag”, meaning a bag that does not weigh much. Calling it a bright bag changes the description to something else. The same substitute has followed the wrong meaning.
The dictionary lists these different senses of light. A word can share one meaning with another word without sharing every meaning it has.
The same idea helps with slang. Mogging and outshining someone can describe a similar comparison, with a different tone.
Meaning, sound and opposites
Synonyms are about meaning. Antonyms point towards opposites, such as open and closed in a description of a door.
Homophones share a pronunciation. Sea and see sound alike, but one names a body of water and the other can describe using your eyes. Sounding the same does not make them synonyms.
Try the substitute in place
When you meet a possible synonym, read the whole sentence first. Decide which meaning the writer intends, put the candidate word in place and read the sentence again. Has the intended meaning stayed close?
You may find more than one reasonable choice. Nearly the same leaves room for differences in emphasis, and a dictionary entry is a starting point rather than a promise that every swap works.
Keep investigating sounds, letters and meanings in Word detective. Each optional round gives you another sentence or distinction to try.