wǒ mǎi le niúròu.

In this sentence, 了 marks that the action of buying was completed.

So 我买了牛肉 means something like:

• I bought beef
• I have bought beef

It does not exactly equal the English past tense, but in many simple sentences it often corresponds to a completed action in the past.

A helpful way to think about it is:

• 买 = buy / to buy
• 买了 = bought / did buy / have bought

Here, 了 is attached to the verb phrase to show that the action was completed:

• 我 买了 牛肉
• I bought beef

This is very common in Mandarin: verb + 了 + object.

Sentence-final 了 also exists, but it often adds a different nuance, such as a new situation or change of state.

Compare:

• 我买了牛肉。 = I bought beef.
• 我买牛肉了。 = I’m buying/bought beef now, or as for beef, I bought it; often more context-dependent.

For beginners, the safest pattern is:

• Subject + Verb + 了 + Object

Yes, but the meaning changes.

• 我买了牛肉。 = I bought beef.
  Completed action.

• 我买牛肉。 = I buy beef / I am buying beef / I buy beef in general.
  This is more neutral and does not clearly mark completion.

Without 了, the sentence may sound like:

• a habit
• a general statement
• something happening in a broader context

So if you specifically want I bought beef, 了 is the natural choice.

Because 牛肉 here is being used as an uncountable noun or general food item, similar to beef in English.

Chinese usually needs a measure word only when you count or specify an amount:

• 我买了牛肉。 = I bought beef.
  No specific quantity mentioned.

But if you want to say how much, you would usually add a measure expression:

• 我买了一斤牛肉。 = I bought half a kilogram of beef.
• 我买了一些牛肉。 = I bought some beef.

So the sentence is natural because it simply states what was bought, not how much.

Yes. It is made of:

• 牛 = cow / cattle / ox
• 肉 = meat

So 牛肉 literally means cow meat, which is the normal Chinese word for beef.

This is very common in Mandarin food vocabulary:

• 猪肉 = pork
• 鸡肉 = chicken
• 羊肉 = lamb / mutton

So learning 肉 is very useful, because it appears in many meat words.

Yes. This is a very important pronunciation point.

In isolation:

• 我 = third tone
• 买 = third tone

But when two third tones come together, the first one usually changes to a second tone in pronunciation.

So 我买 is pronounced roughly like:

• wó mǎi

Even though it is still written as wǒ mǎi in pinyin.

This is called third-tone sandhi.

So the full sentence is written:

• wǒ mǎi le niúròu

But when spoken naturally, the beginning sounds more like:

• wó mǎi le niúròu

You usually say:

• 我没买牛肉。

Use 没 (méi) or 没有 (méiyǒu) to negate a completed action.

Notice that 了 is normally removed here:

• 我买了牛肉。 = I bought beef.
• 我没买牛肉。 = I didn’t buy beef.

Do not normally say:

• 我不买了牛肉 ✘

Also compare:

• 不买 = do not buy / will not buy / don’t buy
• 没买 = did not buy

So for this sentence, 没买 is the correct negative form.

Yes, very often—if the context already makes the subject clear.

For example, in conversation:

• 买了牛肉吗？ = Did you buy beef?
• 买了牛肉。 = Bought beef. / Yes, I bought beef.

Mandarin often drops subjects when they are obvious from context. But in a standalone beginner sentence, keeping 我 is helpful and natural:

• 我买了牛肉。

So:

• With 我 = clearer, complete sentence
• Without 我 = natural in conversation if the speaker is understood

The word order is:

• 我 = subject
• 买 = verb
• 了 = completed-action marker
• 牛肉 = object

So the pattern is:

• Subject + Verb + 了 + Object

This is similar to English in one important way: the object comes after the verb.

Compare:

• 我买了牛肉。 = I bought beef.
• 他吃了苹果。 = He ate an apple.
• 我们看了电影。 = We watched a movie.

This is one of the most useful basic Mandarin sentence patterns to learn early.