zhè gè yào chī le yǐhòu, bù huì mǎshàng tuìshāo, kěshì wǎnshang yīnggāi huì shūfu yìdiǎnr.

Yes. In 这个药, 个 is the classifier in the pattern 这 + classifier + noun.

Mandarin normally needs a classifier after 这 / 那 / 一 / 两 and so on. So:

• 这药 can occur in casual speech, but
• 这个药 is the standard full form

Even though 药 can appear with more specific words in other contexts, 个 is very common as a general classifier in everyday speech.

Compare:

• 这个药 = this medicine
• 这种药 = this kind/type of medicine
• 一片药 = one pill/tablet
• 一瓶药 = one bottle of medicine

So here 个 is natural and normal.

Yes, 吃 literally means to eat, but in Chinese it is also commonly used for taking medicine.

So:

• 吃药 = to take medicine

This is one of those expressions that English speakers just have to get used to. Chinese often uses 吃 for medicine, especially when it is something you swallow, like pills or tablets.

Other verbs can also be used depending on the type of medicine:

• 喝药 = drink medicine
• 上药 = apply medicine to a wound
• 打针 = get an injection

But 吃药 is a very common everyday expression.

吃了以后 means after taking it or after you take it.

The pattern is:

• V + 了 + 以后 = after doing V

So here:

• 吃了以后 = after taking/eating it

The 了 marks the action of 吃 as completed before the next thing happens. In other words, first you take the medicine, and then we talk about what happens next.

Similar examples:

• 看了以后 = after seeing/reading it
• 到了以后 = after arriving
• 吃了饭以后 = after eating

Without 了, the sentence would sound less natural in this kind of after doing X structure.

Here it means won’t or is not likely to, not cannot in the sense of lack of ability.

会 has several uses in Mandarin, including:

1. know how to

   • 我会说中文。 = I can speak Chinese.

2. will / be likely to

   • 明天会下雨。 = It will probably rain tomorrow.

In this sentence, 不会马上退烧 is talking about what will happen after taking the medicine, so 会 expresses future likelihood or prediction:

• 不会马上退烧 = it won’t reduce the fever immediately / the fever won’t go down immediately

So this is about expected result, not ability.

This is because of a tone change rule.

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

So:

• 不 + 会 (huì) → pronounced bú huì

It is still written bù in pinyin dictionaries and textbooks as the basic form, but in actual speech you say bú huì.

A few more examples:

• 不是 → bú shì
• 不会 → bú huì
• 不对 → bú duì

退烧 means for a fever to go down or to reduce a fever.

• 烧 here refers to fever
• 退 suggests going down, subsiding, or receding

So 退烧 is the natural Chinese expression for bring the fever down / break the fever.

In this sentence, 不会马上退烧 does not mean the medicine is useless. It means the fever will not go down immediately after taking it.

This is a very common medical expression. You may also hear:

• 发烧 = to have a fever
• 退烧药 = fever-reducing medicine

Because 马上 is an adverb, and in Chinese adverbs usually come before the verb or adjective they modify.

So:

• 马上退烧 = to have the fever go down immediately

This matches a common Chinese word-order pattern:

• subject + time/adverb + verb

Examples:

• 他马上来。 = He’s coming right away.
• 我马上做。 = I’ll do it immediately.
• 不会马上退烧。 = It won’t reduce the fever immediately.

For English speakers, the position can feel familiar here, since English also often puts immediately before the verb, but in Chinese this pre-verb position is especially regular.

可是 and 但是 both mean but. In many cases they are interchangeable.

Here, 可是 introduces a contrast:

• the medicine won’t reduce the fever immediately,
• but by evening you should feel somewhat better.

The difference is mostly one of tone and style:

• 可是 often sounds a bit more conversational or spoken
• 但是 can sound slightly more neutral or formal

So this sentence could also use 但是, but 可是 is very natural in everyday speech.

Because the speaker is making a careful expectation, not an absolute statement.

• 应该 = should / ought to / is expected to
• 会 = will / is likely to

Together, 应该会 often means something like:

• should probably
• is expected to
• will likely

So:

• 晚上应该会舒服一点儿 = by evening, you should probably feel a bit better

Using both words makes the prediction sound softer and less absolute than just 会.

Compare:

• 晚上会舒服一点儿。 = You’ll feel a bit better tonight.
  More direct prediction.

• 晚上应该会舒服一点儿。 = You should probably feel a bit better tonight.
  More cautious and natural in medical advice.

In this sentence, 晚上 most naturally means this evening / tonight / by evening, depending on the English translation.

Chinese time words often appear directly before the verb or predicate without a preposition like 在.

So:

• 晚上应该会舒服一点儿 = by evening / tonight, you should feel a bit better

This is normal Chinese word order. You also see:

• 我明天去。 = I’m going tomorrow.
• 他晚上来。 = He’s coming tonight.
• 我们下午见。 = We’ll meet in the afternoon.

You can sometimes use 在 with time expressions, but it is often unnecessary, and here 晚上 by itself is the natural choice.

舒服一点儿 means a little more comfortable or simply a bit better.

• 舒服 = comfortable / feel well
• 一点儿 = a little bit

So the idea is not that the person will be fully well by evening, only that they should feel some improvement.

With adjectives, 一点儿 often indicates a small degree of change:

• 好一点儿 = a little better
• 快一点儿 = a little faster
• 舒服一点儿 = a little more comfortable

In context, the comparison is usually understood:

• compared with right now
• compared with before taking the medicine

Also, 一点儿 is very common in northern-style speech. You may also see 一点 in some contexts, but 一点儿 is very natural here.

The subject is understood from context, and Chinese often leaves it out when it is obvious.

Here, the implied subject is the person taking the medicine, so:

• 晚上应该会舒服一点儿 = by evening, you should feel a bit better

Chinese frequently omits subjects when the listener can easily infer them. English usually requires the subject to be stated, but Mandarin often does not.

This happens in the earlier part too. The sentence does not spell everything out in a fully explicit way like English might. Instead, it relies on context:

• this medicine
• after taking it
• the fever won’t go down immediately
• but you should feel a bit better by evening

A useful way to see it is as a topic + time condition + comment + contrast + second comment structure.

Breakdown:

• 这个药 = this medicine
  The topic

• 吃了以后 = after taking it
  Time/condition

• 不会马上退烧 = won’t reduce the fever immediately
  First comment

• 可是 = but
  Contrast

• 晚上应该会舒服一点儿 = by evening, [you] should feel a bit better
  Second comment

So the sentence flows like this:

• As for this medicine, after taking it, it won’t bring the fever down right away, but by evening you should feel a bit better.

This kind of structure is very natural in Chinese. Instead of building everything around a fixed subject the way English often does, Mandarin often introduces a topic first and then comments on it.