"The dog was killed."
Translation:Mi gaeth y ci ei ladd.
If it helps to remember this word, it appears to be related to English words such as slayed and slaughter - so just think that the dog was slayed. All we need is some way to explain the disappearance of the s.
GPC suggests (and you can choose whether to believe this) that it comes from slaide, the verb-noun of Old Irish slaidid. The soft mutation of sl in Irish is pronounced /hl/, which GPC writes as Welsh ll because they would indeed sound quite similar. In other words you can explain any sl → ll just by assuming it comes from Old Irish. D