We use the Passive when we do not know who did something, or when it is not important to say who did it.
As with Active sentences, we can use different tenses in the Passive, e.g. Present Passive, Past Simple Passive, etc.
Present Passive
The Present Passive is often used to talk about a custom that exists in the present.
A pasta dish is served before the main course.
A special meal is eaten on Friday evening.
= subject + Present Simple of be + past participle
We also use the Present Passive to describe a process such as the method of making and preparing a dish, or talking about ingredients and accompaniments. In this case, we often use the prepositions of or with after the Passive verb.
A salade Marocaine is made of chopped tomatoes.
Schnitzel is served with potato salad.
This dish is cooked with potatoes and carrots.
Be careful to use the correct prepositions after the verb.
It is possible to use other prepositions after the Passive, but these depend on the main verb.
The meat is covered in breadcrumbs.
Relative clauses: who, which, that
We use relative clauses to connect two ideas. There are two types of relative clause.
Non-defining
Look at the following sentences.
I really love sushi. Sushi is our national dish.
By using the pronoun who or which, we can combine the two sentences and avoid repeating the subject.
I really love sushi, which is our national dish.
MAIN CLAUSE RELATIVE CLAUSE
We use which to refer to a thing or things, and who to refer to a person or people.
In this way,
Turkish food is popular among tasters. Tasters visit regions as part of their holiday.
becomes
Turkish food is popular among tasters, who visit regions as part of their holiday.
Note that in the sentences containing which and who, the two parts of the sentences are separated by a comma. The second part of the sentence is called a non-defining relative clause. This is because what comes after the comma is simply additional information, which we do not need in order to understand the whole sentence.
Defining
The second type of relative clause also connects two ideas, but in a different way. Look at the following sentences.
A tagine is a pot. A tagine is used to cook food in.
A tagine is a pot which is used to cook food in.
MAIN CLAUSE RELATIVE CLAUSE
Also
Tasters are tourists. Tasters are interested in the food of the region.
Tasters are tourists who are interested in the food of the region.
MAIN CLAUSE RELATIVE CLAUSE
In this case, the relative clause is not just giving additional information, and does not come after a comma. Instead, it is an essential part of the sentence that helps us to identify which things or people we are talking about.
Note that in a defining relative clause, we can use that instead of which or who.
A tagine is a pot that is used to cook food in.
Tasters are tourists that are interested in the food of the region.
We cannot use that in non-defining relative clauses.
NOT I really love sushi, that is our national dish.
Source: Oxford English for Careers. Tourism 2.