Learn Hardcore Chinese: fùmǔ zài gōngyuán sànbù le yí gè xiǎoshí, juéde shēntǐ hěn shūfu. - My parents walked in the park for an hour and felt very comfortable physically.

The subject 父母 naturally carries over into the second clause 觉得身体很舒服. Native speakers understand that the ones who 觉得 are the 父母, so 他们 is normally omitted.

You could say:

父母在公园散步了一个小时，他们觉得身体很舒服。

This is grammatically fine but a bit heavier; the original is smoother and more typical in Chinese.

What is the function of 了 after 散步? Does it mean past tense?

了 here is the aspect particle marking a completed action, not a tense marker like in English.

Pattern: Verb + 了 + Duration
So: 散步 + 了 + 一个小时 = walked for an hour (and that walking is finished).

Key points:

It does not literally mean past tense, but since completed actions are often in the past, it often corresponds to past tense in English.
Without 了, 父母在公园散步一个小时 sounds off here. It could sound like a plan or a habit, but that is not how Chinese usually expresses it.

So 了 makes us understand this as they walked (and finished walking) for an hour.

Why is 一个小时 after 散步了 instead of before the verb?

This follows a very common pattern in Mandarin:

Verb + 了 + Duration = did something for X time

So we say:

散步了一个小时 (walked for an hour)
等了半个小时 (waited for half an hour)
学习了三年 (studied for three years)

Putting the duration before the verb, like 一个小时散步了, is ungrammatical in this kind of sentence.

You can put a general time expression before the verb, but that is when it means when, not how long, for example:

昨天父母在公园散步
Yesterday the parents took a walk in the park.

But for how long, Chinese likes Verb + 了 + Duration.

Why do we say 一个小时 and not just 一小时? Is 个 necessary?

Both 一个小时 and 一小时 are acceptable and common.

一个小时: more colloquial, very common in speech.
一小时: a bit more concise, sometimes feels slightly more formal or written.

个 here is the general measure word. Many speakers automatically insert 个 in everyday conversation, so 一个小时 may sound more natural in spoken Mandarin, but dropping 个 is fine:

散步了一个小时 ✅
散步了一小时 ✅

Meaning is the same.

Why is 一 pronounced yí instead of yī in 一个小时?

This is tone sandhi (tone change in context).

The basic tone of 一 is first tone (yī), but:

Before a fourth-tone syllable, 一 changes to second tone: yí.
Before 个 (gè, fourth tone), it becomes yí: yí gè.

So:

一个小时 → yí gè xiǎoshí

This is a phonetic rule to make speech flow more smoothly. The written tone mark on 一 is often adjusted in learning materials to show this.

What does 觉得 add here, and how is it different from 认为?

觉得 expresses a feeling or subjective impression.
认为 expresses a considered opinion or judgment.

In this sentence:

觉得身体很舒服 = they feel physically comfortable (a bodily sensation).

If you used 认为 here:

父母…认为身体很舒服

it would sound strange, as if they are logically judging whether their body is comfortable, instead of feeling it. So 觉得 is the natural choice for physical and emotional states:

我觉得累。 I feel tired.
我觉得他很好。 I feel he is very nice.

Why do we need 很 before 舒服? Isn’t 很 = very? Are they saying very comfortable?

In Chinese, adjectives used as predicates usually need something in front of them, and 很 often works as a neutral linker, not always as strong as English very.

身体很舒服。
Literally: body very comfortable, but in many contexts it can just mean (the) body is comfortable.

If you say:

身体舒服。

this can sound slightly incomplete or unnatural in standard Mandarin (though it can appear in some contexts).

So 很 here:

Makes the sentence sound natural.
Often does not strongly emphasize intensity.
Context decides if it feels like very or just (is) comfortable.

If you want to be clearly stronger, you can say 非常舒服, 特别舒服, etc.

Why say 身体很舒服 instead of just 很舒服?

身体很舒服 focuses on physical comfort.

身体很舒服 = Their body feels comfortable; they feel good physically.
很舒服 alone could also be OK (if the context is clear), but it is slightly more vague and less complete as a clause.

By adding 身体, the speaker clearly talks about their health / physical state after exercise, not about something like the environment is comfortable or the chair is comfortable.

So here 身体 clarifies that the comfort is in their body after walking.

What is the overall word order pattern in this sentence?

The basic structure is:

Subject + 在 + Place + Verb + 了 + Duration，+ (Subject) + 觉得 + Object + 很 + Adjective

Applied:

父母 – subject
在公园 – location
散步了一个小时 – verb + aspect + duration
觉得 – feel / think
身体 – what they feel about (their body)
很舒服 – adjective predicate

So the order is:

Who → Where → Did what, for how long → (then) How they felt / what they thought about something.

This matches a very common Chinese pattern:
Subject + Time/Place + Verb Phrase + Comment / Result.

How do we know this happened in the past when there is no past tense form?

Mandarin does not change verb forms for tense. It uses:

Aspect particles (like 了)
Time words (like 昨天, 刚才, 明天, etc.)
Context

Here, the feeling 觉得身体很舒服 is a result of 散步了一个小时. The 了 indicates the walking is a completed event, so in English we naturally translate it using past tense:

They walked in the park for an hour and felt comfortable.

So the past meaning comes from 了 and the logical sequence, not from an inflected verb form.

Is 散步 a single verb or a verb plus an object? Can it be split?

散步 is a verb–object compound:

散 (to stroll, to disperse)
步 (step)

Historically it is verb + object, so in some structures it can be split, but in modern usage it is very often treated as a fixed verb meaning to take a walk / to go for a stroll.

Common patterns:

散步了一个小时 ✅
在公园散步 ✅

You would not normally say 散了一个小时步 in modern everyday Mandarin; it sounds old-fashioned or literary. So in practice, treat 散步 as one verb and place 了 + duration after it, as in the sentence you have.

Could we move 在公园 to the end, like 父母散步了一个小时在公园?

No; that word order is unnatural in Chinese.

Location phrases with 在 usually go before the main verb phrase:

父母在公园散步了一个小时。 ✅
父母散步了一个小时在公园。 ❌ (sounds wrong)

General Chinese order is:

Subject + (Time) + Place + Verb + Object / Complement

So place usually comes before what you did there, not after the whole verb phrase.