就語形方面,凱爾特語的名詞與形容詞與其上層語並無什麼顯著的改變,但相較於其他實存的印歐語言,海島凱爾特語的動詞就擁有一種獨特且未解的特徵:其動詞的詞形變化會隨著該動詞是否位於一個句子的絕對首位(海島凱爾特語為動主賓即 VSO 語言)或該動詞之前是否有一個動詞前置質詞而有所不同。這樣的特色在古愛爾蘭語中表現得最為突出,但在後來的蘇格蘭蓋爾語和中古威爾斯語裡也有一些相同的例子。
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