How is the addition of 's' used in: für's, morgens früh, etc.? as well as any such ambiguous s's:)

Question

Answer

's' in morgenS change its meaning = in the morning. 
Morgen means on the other hand (tomorrow). 
The same goes for 'Abends' . 
'Abend' means 'evening' 
'AbendS' or 'am Abend' means the same = in the evening. 
When you say 'Samstag' it means 'Saturday'.Adding S here 'Samstags' changes the meaning to 'on Saturdays'. 
"fürs" ist eine Abkürzung für "für das" 

Als Regel gilt : 
Wenn ein -s am Ende ist, liegt meistens ein (ursprüngliches) Adverb wie beim: 
morgens, mittags, abends, montags, freitags, schnellstens. 

Bei Präpositionen liegt dann ein S am Ende der Präposition als eine Abkürzung für den nacher stehenden Artikel " DAS " wie beim: 
fürs , ans 

So it is not really ambiguous ,there are rules there and it does many times change the meaning of the word.

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