In standard Southern British English, /r/ is not normally pronounced before a consonant or at the end of a word.
Examples :
Fi(r)st ca(r) hai(r) borde(r) mo(r)e fou(r)
/r/ is not pronounced when it occurs in the following combinations :
/er/ her refer nerd
/ir/ thirst first bird
/ur/ burn turn churn
/or/ ford north form
/ear/ learn earn yearn
When a word ends with /r/ and the following word begins with an open sound(or a vowel sound), irrespective of the actual letter—the sound is what is important —/r/ is pronounced.(Note for actors : This is also correct in RP.)
To make the linking /r/ sound, add a small ‘R’ sound, but not too heavy.
Examples :
ever after (eveR after)
mother and father (motheR and father)
her eyes (heR eyes)
But also :
The doctor examined her x‐ray (…heR[e]x‐ray)
(Note the pronunciation of ‘x’starts with the /e/ vowel sound.)
They wrote a letter to their MP (…the iR [e] MP)
(Note the pronunciation of ‘MP’ starts with the /e/ vowel sound.)
This is my car. The car is blue (…caRis blue)
One hour. After hours (afteR [h] ours) (the ‘h’ is silent)
So if a word ends in /a/ (pronounced as /a/ or / ə /, the schwa sound) or in /aw/ (the /oh/ɔ: /sound) and the following word starts with any vowel or vowel sound, then an / r / is added by the speaker, even though there is no / r / in the spelling. (Note for actors : This is not done in RP.)
Examples :
India and Pakistan (IndiaRand Pakistan)
Vodka and tonic (vodkaRand tonic)
Law and order (lawRand order)
You will find this phenomenon most often with the word ‘and’ if preceded by a word ending in one of the sounds mentioned above.
Other examples :
Has Leah ever been here? (leahRever been)
I like the idea of visiting Japan (ideaRof)
Please note, it is not absolutely necessary to add the intrusive / r /, but it is very common and is an authentic key sound of the Modern standard British English accent.
Peter in the store over at our old but rather amazing street corner in our area has the air of a more aggressive and better equipped salesman, as he and the other owner Oscar are known for all their amazing goods and their awfully hateful proclamations on sight of business competitors: ‘There are our enemies!’
Maria and Leah and I, talked about the idea of studying law in London over a vodka and tonic