Why is there only a "個" in phrases like打個電話 and 你是個好孩子?
I learned that 個 is a measure word, so I've only seen it with numbers in front of it. But why is there no ㄧ in these two sentences? And why do we even need 個--wouldn't the sentences be ok just as 打電話 and 你是好孩子?

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在你举的这两个例子里，你可以认为“個”＝abbreviation for “一個” 
在口语中，“一個”中的“一”经常会被省略。但是注意，即使在口语中，也不是在所有的时候“一個”中的“一”都可以省略的。

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Please notice that the 一 in 一个 can be omitted only when the thing after the phrase is an object. In other cases, such as before the subject, 一个 should not be written as "个". For instance:

一个人的世界很孤独。

You can't write it as:

个人的世界很孤独。

Other examples are:

一个人在酒吧喝酒。
一个苹果从树上掉了下来。

And, don't omit any number larger than one in front of "个" at any time!!

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首先另一重點在"打"一字 
"打" 在中文裡頭 是個泛意詞 
該字的解釋多達二十個 
"打"字所配合的動量詞不同 意義也就不同 

打"個"電話 指的是在電話機上按鍵 作通話的預備 
這時"打"作撥動 按捺 講 (打方向盤 打舵 打算盤) 

打"通"電話 指的是在電話線路的兩端互通訊息 
這時"打"作發出 講 (打電話 打信號 打電報 打雷) 

若要強調通話內容 打"通"電話較恰當 

在這二例中 "個" "通" 都是量詞 
當數詞為"一"時 通常可略去 
而"你是個好孩子"則屬於此語法 
其他常見量詞 如 張 條 隻 都可適用此規則 

得以省略"一"的場合 也要看文意 
某些量詞前若省略"一"會使得語感變得怪異 
(這時 "這" "那"的指示不可少) 
如 一輪明月 是熟知的固定詞彙 
一輪明月高懸於夜空 
若說成 這輪明月高懸於夜空 
則明月距離我們遙遠的感覺就消失了 
(因為"這"是近稱, "這"與"高懸"語感衝突)

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I think the 一 is optional in the phrase 一个.

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Another instance when the '一‘ is not used is:

这个 (instead of 这一个)
那个 (instead of 那一个)

I like when we are able to take short cuts and type or say less characters in Chinese.  The language is already so complicated as it is.

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