"Thank you, grandfather!"
Translation:Tapadh leibh a sheanair!
English has two features which make it unusual: firstly there is no polite plural; secondly, the second person singular and second person plural has collapsed to only a second person "You", although some dialects retain the distinction. SO how does this help? Gaidhlig makes both distinctions, so if you mean only a singular 'you' AND you are INFORMAL with them, use 'tu' and singular second person forms such as LEAT, ORT, AGAD...etc If you are polite or you are talking to many, then you sibh and plural forms, such as LEIBH. The person is shewing respect to his grandfather, by choosing to use the formal, polite, plural form. I hope this helps, because it is throughout Gaidhlig and is found in other languages.
it is a vocative participle, in short. So, if you address someone, you use 'a' before, which causes lenition of the name, if possible. So, Calum is the normal case of the name, you ask how Calum is, you use 'a Chalum'. The addressing 'a' followed by lenition of Calum. Exception, Gaidhlig drops the 'a' before a name with a vowel, Ciamar a tha thu Iain? Iain is addressed but 'a' is dropped because of a leading vowel. Lastly, not all names are capable of lenition or rather letters.