Sentences in English contain clauses and phrases.
The difference between clauses and phrases, as well as how to use them, is not always clear for learners of English.

CLAUSES :

A clause is a part of a sentence with a subject-verb unit that can function as a complete sentence on its own.

A clause creates a complete thought and can contain one or more phrases.
There are different types of clauses:

• Main (or independent) clauses
• Subordinate (or dependent) clauses
• Relative clauses: defining and non-defining

MAIN (or independent) CLAUSE:
A main clause is a group of words containing a subject and a verb that can form a complete sentence on its own.

• I will save my work before I turn off the computer.
• You can decorate the room however you like.
• Don’t hesitate to contact me if you need more information.

SUBORDINATE (or dependent) CLAUSE :
A clause that adds more information to complete the main clause.
A subordinate clause cannot stand alone as a complete sentence. It is linked to the main clause by a subordinating conjunction.

• I know how to swim because my dad taught me.
• I can't buy a car until I have enough money.
• Since it's a sunny day today, we will go to the beach.

RELATIVE CLAUSE:
Relative clauses give us information about the person or thing mentioned. There are two kinds of relative clauses : defining and non-defining.

1) Defining relative clause:
A defining relative clause adds essentiel information to define or identify the person or thing we are talking about.
To start a defining relative clause, we use a relative pronoun or adverb : who, whom, whose, that, which, when, where, that or why.
No commas are used around defining relative clauses.

• The red car belongs to the man who lives next door.
• She will confide in someone whom she can trust.
• Here is my friend whose mother is a doctor.
• The house that my parents bought is quite small.
• They visited a church which was built in the 14th century.
• There’s the hotel where we stayed last year.
• We eat outside on the patio when it's sunny.
• The reason why I came here today is not important.

2) Non-defining relative clause:
A non-defining relative clause gives extra non-essentiel information concerning who or what we are speaking about.
We use a relative pronoun or adverb (who, whose, whom, which, where (but not ‘that’) with a non-defining relative clause.
Commas are placed around a non-defining relative clause to separate it from the rest of the sentence.

• My grandmother, who is dead now, came from Ireland.
• My younger sister, whose husband works in Boston, is a nurse.
• Sarah, with whom I work, is expecting a baby.
• Spiders, which are a type of insect, can be venomous.

PHRASES :
A phrase is a group of words, without a subject-verb unit, that provides extra information.
Phrases do not function on their own.
They act as nouns, adjectives, adverbs and so on. Here are some examples:

• Noun phrase: "A smart young lady arrived."
• Verb phrase: "The man was running quickly."
• Adverbial phrase: "He spoke loudly and rudely."
• Prepositional phrase: "The file was on top of the desk."
• Time phrase: "Once a month I go to the hairdresser's."

SENTENCES :
Sentences in English can be brief or more complex.
They can contain any number of clauses and phrases combined together.

Sentences with clauses:
• She arrived. (one clause)
• She arrived and he greeted her. (2 clauses)
• She arrived, he opened the door and he greeted her. (3 clauses)
• She arrived, he opened the door, he greeted her and he invited her in. (4 clauses)
  

Sentences with phrases:
• A group of four people entered the room. (noun phrase)
• Children are usually full of life. (adjectival phrase)
• He arrived 30 minutes ago. (time phrase)
• She acted with determination. (adverbial phrase)
• I’d like to travel around the world (prepositional phrase)
  

Example sentence with 3 clauses and 4 phrases:
• Cakes and pastries (noun phrase) were put on the table (prepositional phrase), and the children ate them quickly and noisily (adverbial phrase) until there were only a few crumbs (noun phrase) left.