wèile bú ràng yóuxìjī yǐngxiǎng chéngjì, gēge juédìng měitiān zhǐ wán bàn gè xiǎoshí.

为了 introduces a purpose or goal. A very common pattern is:

为了 + purpose, main clause

So 为了不让游戏机影响成绩 means in order not to let the game console affect grades.

You can think of 为了 as meaning in order to or for the sake of.

This is a pronunciation change called tone sandhi.

Normally, 不 is fourth tone: bù.
But when 不 comes before another fourth-tone syllable, it changes to second tone: bú.

Here:

• 让 = ràng (4th tone)

So:

• 不让 is pronounced bú ràng

The character is still 不. Only the pronunciation changes.

Because this is a statement of purpose, not a command.

• 不让 = not let / prevent
• 别让 = don’t let... (more like telling someone not to do something)
• 不要让 = also don’t let..., often more explicit or formal as a command/advice

In this sentence, the brother is making a decision so that the game console does not affect his grades. So 为了不让... is the natural choice.

Here, 让 means to let / to allow / to cause.

A useful pattern is:

让 + noun + verb

So:

• 让游戏机影响成绩 = let the game console affect grades
• 不让游戏机影响成绩 = not let the game console affect grades

In English, this sounds a bit like prevent the game console from affecting grades.

So 让 is connecting 游戏机 with the action 影响成绩.

Because 影响 is often used as a transitive verb in Chinese. That means it can take its object directly.

So you can say:

• 影响成绩 = affect grades
• 影响身体 = affect health
• 影响心情 = affect mood

You do not need an extra word like to or on here.

There is also another common pattern:

• 对...有影响 = to have an effect on...

For example:

• 对成绩有影响 = have an effect on grades

Both are natural, but this sentence uses the direct verb pattern.

Chinese often uses family titles very naturally where English would often use a pronoun or a name.

So 哥哥 means older brother, and using it directly sounds normal in Chinese.

Depending on context, it may mean:

• the speaker’s older brother
• someone already known in the conversation as older brother

Chinese does this a lot with words like:

• 妈妈 = mom
• 爸爸 = dad
• 姐姐 = older sister
• 弟弟 = younger brother

So 哥哥决定... sounds very natural.

Because 每天 describes the action 玩, not the verb 决定.

The structure is basically:

哥哥 决定 [每天 只 玩 半个小时]

So the part after 决定 is what he decided to do:

• 每天 = every day
• 只玩半个小时 = only play for half an hour

If you moved 每天 earlier, it could sound like it modifies 决定 instead, which is not the main idea here.

So this placement makes it clear that every day goes with play.

只 means only.

In Chinese, adverbs like 只 usually go before the verb phrase they modify.

So:

• 只玩半个小时 = only play for half an hour

Here, 只 limits the amount of time spent playing.

Compare:

• 每天玩半个小时 = play for half an hour every day
• 每天只玩半个小时 = only play for half an hour every day

The second one emphasizes restriction.

Because in Chinese, duration usually comes after the verb.

A common pattern is:

verb + duration

So:

• 玩半个小时 = play for half an hour
• 学两个小时 = study for two hours
• 睡八个小时 = sleep for eight hours

This is different from English, where learners may want to say something like for half an hour play, but that is not how Chinese normally works.

Yes, 半个小时 and 半小时 are both natural.

• 半个小时 = half an hour
• 半小时 = half an hour

个 is acting as a measure word here, and 半个小时 sounds very common and conversational.

A useful comparison:

• 半个小时 = half an hour
• 一个半小时 = one and a half hours

So be careful not to confuse half an hour with one and a half hours.

Because the sentence is talking about a decision/plan, not a completed action.

• 决定 here means decides / decided to
• 每天只玩半个小时 describes a rule or habit he plans to follow

If you said 玩了半个小时, that would usually describe a completed event:

• 他今天玩了半个小时。 = He played for half an hour today.

You could sometimes say 决定了 in another context to stress that the decision has been made, but in this sentence plain 决定 is perfectly natural.

A helpful way to break it down is:

为了 + purpose, subject + 决定 + time + adverb + verb + duration

In this sentence:

• 为了不让游戏机影响成绩 = purpose
• 哥哥 = subject
• 决定 = main verb
• 每天 = time
• 只 = adverb
• 玩 = verb
• 半个小时 = duration

So the sentence is built very systematically, and once you see the pattern, it becomes much easier to understand and reuse.