Meaning of -ER in English
You add ~ to many short adjectives to form comparatives. For example, the comparative of ‘nice’ is ‘nicer’; the comparative of ‘happy’ is ‘happier’. You also add it to some adverbs that do not end in -ly. For example, the comparative of ‘soon’ is ‘sooner’.
You add ~ to verbs to form nouns which refer to a person, animal, or thing that does the action described by the verb; for example a ‘reader’ is someone who reads and a ‘money-saver’ is something that saves money.
You add ~ to words to form nouns which refer to a person who is associated or involved with the thing described by the word; for example a ‘pensioner’ is someone who is entitled to a pension.
You add ~ to nouns to form nouns or adjectives which refer to things with a particular characteristic or feature; for example a ‘three-wheeler’ is a vehicle with three wheels.
You add ~ to words to form nouns which refer to a person with a particular job. For example, someone who works in a mine is a ‘miner’.
You add ~ to the names of some places to form nouns which refer to a person who comes from that place. For example, someone who comes from London is a ‘Londoner’.
Collins COBUILD. Толковый словарь английского языка для изучающих язык Коллинз COBUILD (международная база данных языков Бирмингемского университета) . 2012