cuspair doirbh a-rithist? Mas e ur toil e?

[Níall Beag]

Ach sin an aon rud: Anglo-Danish.

[Thrissel]

Le co-thuiteamas, nochd an cuspair san Litir do Luchd-ionnsachaidh an seachdain sa chaidh:

Do you notice the girl addresses her father with the “sibh” (plural, respectful) form of pronoun rather than “thu”. This shows him respect because of his age and relationship to her. He, on the other hand, addresses her as “thu”. The man uses sibh with the king, whereas the king uses thu with the man. This traditional usage is perhaps not as strong as it once was, but it is still important. It is one of the ways we express politeness in our use of language. The Gaelic equivalent for “please” – mas e do thoil e – is only infrequently used. We don’t have the “magic word” in Gaelic but “magic phraseology”.

http://downloads.bbc.co.uk/alba/foghlam ... tir560.pdf

[Níall Beag]

Dè an diofar, sa Bheurla, eadar "Sit down" is "Do sit down"? A bheil diofar ann?

You're looking for the wrong equivalents. Compare "sit down" and "take a seat" and you've got a clear difference in politeness, even though both are imperative (command) forms.