个 (gè) is the most commonly used measure word. It can be used in a pinch for any noun if you can't think of a more precise measure word. (Although you might not sound quite as smart, you'll still get your point across). Also, for many nouns, 个 (gè) is the only correct measure word.
The general structure for 个 (gè) and measure words in general is:
Number + 个 + Noun
Any time you want to state how many of a noun in Chinese, you probably need a measure word. First get used to how they are used with 个 (gè).
yī gè rén
sì gè péngyou
sān gè Píngguǒ shǒujī
three iPhones (lit. “three Apple cellphones”)
wǔ gè xīngqī
liù gè yuè
liǎng gè lǎopo
Shí gè nánrén, qī gè shǎ, bā gè huài.
Ten men: seven are fools, and eight are bad.
[Note: This is a a line from a song.]
Omitting the Number
Verb + 个 + Noun
If the number is one (1), you can omit it and use 个 (gè) by itself. This is similar to "a" or "an" in English, for example in "a person" or "an idiot." (The tone on 个 (gè) is normally somewhat de-emphasized in this usage, but still written as fourth tone. You don't need to stress about it, though.)
Tā shì gè lǎowài.
He is a foreigner.
Wǒ yǒu gè érzi.
I have a son.
Tā shì gè hǎo lǎoshī.
She is a good teacher.
Nǐ xiǎng chī gè bāozi ma?
Would you like to eat a stuffed steamed bun?
Lǎoshī, wǒ yǒu gè wèntí.
Teacher, I have a question.
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If you have any questions about this grammar point, please ask in the class forums!