takusan aruita ato ha nodo ga kawaku node, mazu hitokuti mizu wo nomimasu.

Because あと meaning after is attached to the plain past form of a verb when you mean after doing something.

• 歩いたあと = after walking
• 食べたあと = after eating
• 勉強したあと = after studying

So:

• たくさん歩いたあと = after walking a lot

Using 歩くあと would be incorrect here.

In 歩いたあとは, the は marks as for after walking a lot... and sets that time/situation as the topic.

So the sentence is structured a bit like:

• As for after walking a lot, my throat gets dry, so...

This は often gives a general or habitual feeling, like the speaker is talking about what usually happens in that situation.

Compare:

• 歩いたあと、 ... = after walking...
• 歩いたあとは、 ... = as for after walking..., after walking, typically...

The version with は sounds slightly more like a general statement or a personal routine.

Because 渇く is an intransitive verb. It describes a state: something gets thirsty / becomes dry.

So the thing experiencing that state is marked with が:

• のどが渇く = the throat gets dry / one gets thirsty

You do not say のどを渇く.

This is similar to other intransitive patterns:

• ドアが開く = the door opens
• 気分が悪くなる = I feel sick / my condition gets bad

In natural English, のどが渇く is usually translated as to get thirsty.

That is just a natural Japanese way to express thirst.

• のどが渇く literally points to the throat
• In English, we usually say I get thirsty or I’m thirsty

Japanese often expresses physical sensations this way, using body parts or conditions rather than directly saying I.

So even though the literal image is my throat gets dry, the natural meaning is simply:

• I get thirsty
• I’m thirsty

Because the sentence is not just reporting two completed past actions in sequence. It is describing what happens after walking a lot.

• 歩いた = walked
• のどが渇く = the throat gets dry / I get thirsty

The non-past 渇く can express something that happens in that situation, especially a general tendency or normal result.

So the meaning is like:

• After I walk a lot, I get thirsty...

If the speaker wanted to describe one specific past event more firmly, they might also use past forms in the rest of the sentence depending on context. But as written, it sounds natural as a general/habitual statement.

ので means because / so, and it gives a slightly softer, more explanatory tone than から.

Here:

• のどが渇くので = because I get thirsty / since I get thirsty

So the sentence says:

• Because I get thirsty, I first drink a sip of water.

Difference in nuance:

• から can sound more direct, assertive, or personal
• ので often sounds more neutral, explanatory, or polite

Compare:

• のどが渇くから、飲みます。 = I’m thirsty, so I’ll drink.
• のどが渇くので、飲みます。 = Since I get thirsty, I drink.

Both are correct, but ので fits well in calm explanation.

まず means first or to begin with.

Here it shows the first thing the speaker does:

• まず一口水を飲みます = first, I drink a sip of water

It often appears when someone is describing a sequence or routine:

• まず手を洗います。 = First, I wash my hands.
• まず宿題をします。 = First, I do my homework.

一口 literally means one mouthful or one sip / one bite, depending on what you are consuming.

In this sentence:

• 一口水を飲みます = drink one sip of water

Examples:

• 一口飲む = take a sip
• 一口食べる = take one bite

It is a common counter/measure word for a small amount taken into the mouth at one time.

In this sentence, 一口 is functioning adverbially: it tells you how much water is drunk.

So:

• 一口水を飲みます = drink a sip of water

Japanese often allows quantity expressions to appear without an extra particle before the object:

• 水を一口飲みます
• 一口水を飲みます

Both are understandable.

You may also see:

• 水を一口飲みます — probably the most straightforward for learners
• 一口の水を飲みます — more literally drink one mouthful of water, but this can sound a bit more formal or descriptive

So the version in your sentence is a natural compact way to express quantity.

Yes. A very common word order is:

• まず水を一口飲みます。

This may feel easier for learners because it clearly keeps 水を together as the object.

Both of these are natural:

• まず一口水を飲みます。
• まず水を一口飲みます。

The difference is mostly one of word order and emphasis, not basic meaning.

たくさん means a lot and modifies the verb 歩いた.

So:

• たくさん歩いた = walked a lot

It works like an adverb here.

Other examples:

• たくさん食べた = ate a lot
• たくさん勉強した = studied a lot

You can think of it as answering how much did you walk? → a lot.

Yes, the subject is omitted, which is very normal in Japanese.

The sentence does not explicitly say I, but from context we understand:

• (I) walk a lot
• (I) get thirsty
• (I) drink a sip of water

Japanese often leaves out subjects when they are obvious from context. In English, we usually have to supply them.

So a natural English translation would include I, even though Japanese does not.

Yes, both are possible.

• のど
• 喉

In everyday Japanese, body-part words are often written in kana, especially in beginner-friendly materials. のど is very common and easy to read.

So there is no special meaning difference here; it is mostly a writing choice.

It breaks down like this:

• たくさん歩いたあと は = as for after walking a lot
• のどが渇くので、 = because I get thirsty
• まず一口水を飲みます。 = first I drink a sip of water

So the flow is:

1. situation: after walking a lot
2. reason: I get thirsty
3. action: I first drink some water

That kind of cause-and-result structure is very common in Japanese.