就语形方面，凯尔特语的名词与形容词与其上层语并无什么显著的改变，但相较于其他实存的印欧语言，海岛凯尔特语的动词就拥有一种独特且未解的特征：其动词的词形变化会随着该动词是否位于一个句子的绝对首位（海岛凯尔特语为动主宾即 VSO 语言）或该动词之前是否有一个动词前置质词而有所不同。这样的特色在古爱尔兰语中表现得最为突出，但在后来的苏格兰盖尔语和中古威尔士语里也有一些相同的例子。
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