What is the correct way to put the apostrophe. the doubt is with "stadium" because it starts with 's'.
"United's stadium" or "United' stadium"
PS: I need to know why (with grammatical explanation and reference)
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"United's stadium" is the correct spelling, since United is a (one) company and hence thought of as one (singular).
To form the possessive, add apostrophe + s to the noun. If the noun is plural, or already ends in s, just add an apostrophe after the s.
The possessive clitic is almost always 's. The only exceptions are plurals already ending in -s (eg the boys' caps), and some names ending in -s (eg James' book).
So there is always an "s" either before or after the apostrophe (rarely both).
Those rare plurals which don't end in -s take the usual 's (eg the children's caps), so even if you choose to treat United as plural (which British English often does) the possessive clitic is still 's.