Learning and using synonyms
Yeon-Ju from South Korea writes:
  People use synonyms to avoid repeating the same word. For example:

This hotel is so expensive. It's very pricey.

What is the difference in meaning between expensive and pricey? When should I use synonyms?

Roger Woodham replies:
expensive / pricey
Synonyms are words with the same or sometimes slightly different meanings. Alternatives are sometimes used in the same context with little or no difference as in your example, Yeon-Ju, except that pricey is a bit more informal than expensive. Virtually anything that costs a lot of money may be considered expensive or pricey.
keen / eager
I am always keen / eager to introduce synonyms in this way in the examples of use that I quote on the learnit pages. In this example, keen and eager are very close in meaning and may be used interchangeably.

Finding alternatives with the same or similar meaning is undoubtedly a good way of expanding your vocabulary and use of English, Yeon-Ju, but we have to be careful.
disgusting / appalling
These synonyms are quite close in meaning, but not as close as in the previous pairs. Compare the following:
The food they served at John and Paula's wedding reception was disgusting.
The food they served at John and Paula's wedding reception was appalling.
The service at this hotel is disgusting.
The service at this hotel is appalling.
Both adjectives are possible in both contexts, but disgusting is perhaps more appropriate to the first context as it suggests that the food was highly unpleasant to the taste. Appalling is perhaps more appropriate to the second context as it suggests that the service was generally unpleasant, shocking, offensive and unacceptable.
pretty / good-looking / beautiful
These three synonyms, indicating someone or something that is pleasing in appearance, are also quite close in meaning, but use is restricted:
It was a beautiful summer's day.
She was wearing a pretty polka-dot bikini.
With his jet-black hair and high cheekbones he appeared unusually good-looking.
A summer's day cannot be pretty or good-looking. A bikini is not substantial enough to be called beautiful (whereas an attractive wedding dress we would describe as beautiful). Only people, of either sex, can be described as good-looking and men are not usually thought of as pretty or beautiful.
What we learn from this is that words sometimes occur together, or collocate with each other, in fairly fixed ways.
verb + adverb
Certain verbs tend to be used with certain adverbs.
If you think hard / carefully about it, you'll realise that I'm right.
(Not: If you think strongly / powerfully / precisely?)
If I remember correctly / rightly, you were not there at the time.
(Not: If I remember exactly / precisely / truly?
If you truly / really love me, you'll turn down that job in Norway.
(Not: If you purely / justly / rightly / precisely love me?)
adjective + noun
Certain nouns tend to occur with certain adjectives:
It came as a complete surprise to me when she married him
(Not: It came as a comprehensive / full / entire surprise to me?
He carried out a full / comprehensive market survey before launching the product.
(Not: He carried out a complete / all-embracing market survey?
(And not: …before discharging / dispatching / propelling the product.)
verb + noun
Certain Verbs and nouns habitually occur together.
If you eat chocolate before a meal, it will spoil / ruin your appetite.
(Not: …it will damage / harm / suppress your appetite.)
The government has recently conducted / carried out a survey on the causes of obesity in children.
(Not: The government has fulfilled / administered / run a survey?
I can't change my eating habits so I shall continue to eat junk food.
(Not: I can't alter / amend / modify my eating habits?
learning and using synonyms
When you are learning new words it is always a good idea to learn them in the contexts in which they are used and the typical collocations that go with them.
The Ritz. A pricey place to stay?