I know you can use it to shorten to zu dem, but is it also used for the indefinite article?
And is zur a different matter?
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The contractions zum, zur and also im, am, etc. cannot be expanded in any other way than with the definite article. The indefinite articles will not form contractions of that form.
In spoken language, many will shorten the indefinite articles separately, often to ’nem (for einem) or ’ner (for einer). Bavarian goes even further by saying am or a(ra), respectively. However, adding those onto zu will never create zum, only zu ’nem or zu am.
Similarly, in English we’re can never mean we were, only ever we are, even though both verb forms end on the same letters.