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used when we want to talk about a person or thing for the first time or when other people may not know who or what they are

ein

used before a singular noun that starts with a vowel sound, when we are not talking about a specific person or thing

ein

used when referring to a person or thing that was previously mentioned or one that is identified easily

der

used to show that there is none or almost none of something

keine

not any of the members of a group of people or things

keiner

one or some of a thing or number of things, however large or small

irgendein

used to indicate the absence or lack of something

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