不可數名詞點解可以加a/an

Influence 1. 影響力; 勢力, 權勢[不可數名詞]Mr. Smith is a man of influence in this town. 史密斯先生是這個鎮上有權勢的人物。2. 有影響的人(或事物); [可數名詞] A parent has a great influence to their childrenPity 1. 憐憫; 同情[不可數名詞]The villagers took pity on the hungry travelers and gave them hot food. 村民們很同情饑餓的旅行者, 端出熱的食物給他們吃。She never asked for pity before. 她過去從未乞求憐憫。2. 可惜的事, 憾事[可數名詞][+(that)]It's a pity he missed the opportunity to travel abroad. 他錯過了去國外旅遊的機會, 真是可惜。

You may add some words of measurement to the uncountable nouns.
For example,
a cup of tea, a bottle of water, a loaf of bread, an inch of soil, a herd of sheep

Influence ( 是抽象名詞 abstract noun)

It is a pity that a parent has a great influence on their children

不可數名詞可以加 a/an 如果有一個形容詞在前

Example:
rice (是不可數名詞)

a rice (incorrect)

some rice (correct)

a lot of rice (correct)