Hi Erchenko Viktor, Before I get to your question, I would like to comment that it sounds unnatural to say "The boyfriend of my sister". We tend to use "the X (noun) of Y (noun)" for parts of inanimate objects (e.g. the timer of the machine, the gear of the truck, the settings of the computer). It would be more natural to say "My sister's boyfriend". Secondly, there is a difference between "persuade" and "convince". Your sister's boyfriend is trying to make you believe and agree with his opinion, so he is trying to "convince" you. After someone manages to "convince" you, they can "persuade" you to do something. Your sister's boyfriend is discussing the specific concept of "family" as a unit of society. Therefore, the specific article (i.e. the) should be used. As an analogy, someone can try to convince me that THE car will be irrelevant in the future when we are able to travel in flying pods. We are not referring to a specific car here; rather, we are referring to the specific mode of transport using cars. "An" is used to specify "absolutely", not "institution". It is used when the pronunciation of the next word begins with a vowel sound (in the case of "absolutely", it starts with a vowel /æ/ sound). Sometimes, an adjective (or multiple adjectives) comes between the article and the noun. Despite this, the rule of using "an" when the next word is pronounced with a starting vowel sound still holds. Other examples: An irritating boy --> "irritating" starts with a vowel /ɪ/ sound // An old-fashioned institution --> "old" starts with a vowel / oʊ/ sound. I hope this helps you.

Do I understand this correctly: The example sentence: "The boy-friend of my sister tries to persuade me that the family is an absolutely useless and old fashioned institution." And you are asking why "the" is needed before the word "family" and "an" is needed before "institution"? Ok let's start with the article "the" and "family". "The" is a definite article and is used when discussing a particular thing. "The car my brother bought is a lemon." I'm talking about a particular thing. In your example, the particular thing that is being discuss is the institution "family". I believe the confusion is coming from the idea that the sentence isn't discussing a particular family; thus the article wouldn't be needed. However, think more generally. it isn't a family, but an institution that is being discussed, and that institution is "family." Therefore the artifice is needed because you are discussing a definite institution. Regarding the "an". "An" is an indefinite article, and is paired with non specific things. (Remember "A" and "An" are the same, and "An" comes before words beginning with a vowel sound.) I thought I'd eat an apple before sleep. In this sentence I'm not discussing a particular apple. In your example, Institution at the end of the sentence is a general topic being discussed. thus, it takes the an. Recap, In your sentence: Family is the specific institution being discussed Institution is the general topic being discussed. I hope this is helpful and I didn't ramble too much.

"the" is the definite article. You intend a definite family. The concept of the grouping of people called the family. "an" is one form of the indefinite article. It is just one form of institution, out of many possible ones. . You could rewrite it the boy-friend of my sister tries to persuade me that a family is an absolutely useless and old fashioned institution. But the meaning is slightly different. The idea is having a family rather than not having a family.