Anns an or ann?

Anns no ann an?

ann an
in

Use  ann an  for place names  without the definite article.

ann an Dùn Dè
in Dundee
ann an Inbhir Nis
in Inverness
ann an Dùn Èideann
in Edinburgh
ann an Alba
in Scotland
Cò ris a bha an aimsir coltach ann an Inbhir Nis?
What was the weather like in Inverness?
ann am -  in 

Use  ann am  for place names  without the definite article  which begin with  B,MF  or  P.   

ann am Peairt
in Perth
ann am Port Rìgh
in Portree
ann am Baile a' Mhanaich
in Balivanich
Cò ris a bha an aimsir coltach ann am Baile a' Mhanaich?
What was the weather like in Balivanich?
anns an
in the
anns a’
in the

We’ve learnt lots of countries which take the definite article  An | A’.  

They take  anns  instead of  ann an.  

anns an Spàinn
in Spain
anns an Ostair
in Austria
anns a’ Chuimrigh
in Wales
anns a’ Phòlainn
in Poland
Cò ris a bha an aimsir coltach anns an Ostair?
What was the weather like in Austria?

Remember that Gaelic nouns take a gender, and this is true of places too. Countries are generally feminine, but we will look at a couple of masculine place names which take the definite article. 

We’ve already met some changes in masculine place names when we learnt how to say where we are from.  With place names that start with a vowel, we lose the t- .

An t-Òban
Oban
anns an Òban
in Oban
Cò ris a bha an aimsir coltach anns an Òban?
What was the weather like in Oban?

With place names that start with a consonant, we lenite the consonant, and the An (the) becomes A’ .

An Gearasdan
Fort William
anns a’ Ghearasdan
in Fort William
Cò ris a bha an aimsir coltach anns a’ Ghearasdan?
What was the weather like in Fort William?

The  an  changes to  a’  and the word lenites (or takes an h after the first letter). 

The fancy title for when a noun follows a preposition is the dative case (or the prepositional case).

Here is a summary table: 

Feminine place names start with   With an article  Example / eisimpleir  In the dative  Example / eisimpleir 
b, c, g, m, p A + lenition  A’ Ghearmailt  anns anns a’ Ghearmailt 
a, e, i, o, u An An Eadailt anns anns an Eadailt 
d, l, n,r , sg,  sm,  sp and st, t An An Spàinn anns anns an Spàinn 
S+vowel An t- An t-Suain anns  anns an t-Suain 
An Fh  An Fhraing  anns  anns an Fhraing 
Masculine place names start with      
b, c, g, m, p Am / An  An Gearasdan anns an + lenition anns a’ Ghearasdan 
a, e, i, o, u An t- An t-Eilean anns  (lose t-)anns an eilean 
d, l, n, r,  sg,  sm, sp and st, t An An taigh anns  anns an taigh 
S+vowel An t- An Sruthan anns an t-  anns an t-Sruthan 
Am F Am Fàsach Anns an + lenition anns an Fhàsach