maekago, nidai, doroyoke, tyeenkabaa, ryouritusutando ga aru. benrina wake da.

It is basically:

[list of things] が ある。便利な わけだ。

So the first sentence states that certain things exist / are attached / are present.
The second sentence gives the speaker’s conclusion: that explains why it’s convenient or no wonder it’s convenient.

A more structural breakdown:

• 前かご、荷台、泥よけ、チェーンカバー、両立スタンド = a list of items
• がある = there is / there are
• 便利なわけだ = that’s why it’s convenient / no wonder it’s convenient

The subject or topic being talked about is omitted, but it is understood from context, probably something like this bicycle.

Japanese often lists several nouns and then puts one particle after the whole list.

So:

• 前かご、荷台、泥よけ、チェーンカバー、両立スタンド がある

means something like:

• There is a front basket, a rear rack, mudguards, a chain cover, and a double-leg stand.

The が applies to the whole list, not just the last item.

This is very natural in Japanese. English often repeats structure less directly, but Japanese is happy to say A, B, C, D, E がある.

が marks the thing that exists in an ある sentence.

A very common pattern is:

• N がある = there is N / N exists

So here, the listed items are the things that exist or are present.

If you used は, the nuance would change. For example:

• 前かごはある could suggest as for the front basket, there is one or contrast it with something else.

But in this sentence, the speaker is simply stating that these features are present, so がある is the standard choice.

Good question. ある literally means to exist or to be for inanimate things, but in context it can also mean that something has those features.

So in a sentence like this, 前かごがある can naturally mean:

• it has a front basket
• there is a front basket on it

You are right that ついている is also possible when you want to emphasize that something is physically attached:

• 前かごがついている = it has a front basket attached

The sentence with ある is a little simpler and more neutral: it just says these features are present.

Japanese often leaves out things that are clear from context.

This sentence does not explicitly say what has these parts, but it is almost certainly something like:

• この自転車は ...
• その自転車は ...
• このモデルは ...

So the full idea is probably:

• This bicycle has a front basket, a rack, mudguards, a chain cover, and a double-leg stand. No wonder it’s convenient.

Japanese does this constantly. If the topic is already known, it is usually omitted.

Because 便利 is a な-adjective.

When a な-adjective directly modifies a noun, you use な:

• 便利な自転車 = a convenient bicycle
• 静かな部屋 = a quiet room

Here, わけ is a noun, so 便利 modifies わけ, which gives:

• 便利なわけだ

You can think of it as:

• the reason/explanation is that it is convenient
• it follows that it is convenient

So the な is required because 便利 is describing the noun わけ.

わけだ is used when the speaker presents something as a natural conclusion, explanation, or realization.

In this sentence, it means something like:

• that explains why it’s convenient
• so that’s why it’s convenient
• no wonder it’s convenient

The idea is not just it is convenient, but rather:

• Given all those features, it makes sense that it’s convenient.

So わけだ adds a feeling of logical conclusion.

便利だ simply states a fact or opinion:

• It’s convenient.

便利なわけだ adds the nuance of explanation or conclusion:

• That’s why it’s convenient.
• No wonder it’s convenient.

So the difference is:

• 便利だ = plain statement
• 便利なわけだ = statement + reasoning behind it

In this sentence, the speaker first lists the useful features, then concludes 便利なわけだ. That logical flow is exactly why わけだ is used.

Sometimes, but not always in a simple one-to-one way.

わけ often relates to:

• reason
• explanation
• circumstances
• the conclusion that follows from the facts

In 便利なわけだ, it is not just a dictionary-style reason. It is part of the set phrase 〜わけだ, which often means:

• it follows that...
• that explains why...
• no wonder...

So translating わけ by itself as reason can help, but the whole expression is more important than the single word.

No. In normal Japanese writing, this would usually be written without spaces:

• 便利なわけだ。

The same is true for the earlier part:

• 前かご、荷台、泥よけ、チェーンカバー、両立スタンドがある。

Spaces are often added in textbooks or beginner materials to make the structure easier to see. They are not normally used this way in standard Japanese sentences.

The commas simply separate the items, like English commas in a list.

Japanese often uses 、 to list things:

• A、B、C がある

This is a straightforward listing style.
Another possible style would be using や or と, but that changes the nuance:

• AとBとC = A and B and C
• AやBやC = A, B, C, and so on

Here, commas are a neutral way to list several concrete features.

Not necessarily. Japanese nouns usually do not mark singular vs. plural the way English does.

For example:

• 泥よけがある
• チェーンカバーがある

In context, you understand the natural meaning. On a bicycle, some parts are conceptually single items, while others may correspond to something English might describe differently. Japanese usually does not worry about explicitly marking number unless it matters.

So the sentence is not focused on grammar of singular/plural. It is simply listing available features.

Yes, it is a compound noun.

Japanese often combines nouns directly to make one longer noun:

• チェーンカバー = chain cover
• 前かご = front basket
• 両立スタンド = double-leg stand / two-legged stand

You do not always need の between nouns. In fact, direct compounding is extremely common, especially for object names, technical terms, and product features.

If you inserted の, it would often sound less like a fixed term and more like a descriptive phrase. The compound form is the normal one here.

Yes. A polite version would be:

• 前かご、荷台、泥よけ、チェーンカバー、両立スタンドがあります。便利なわけです。

Or even more naturally in a connected style:

• 前かご、荷台、泥よけ、チェーンカバー、両立スタンドがあるので、便利なわけです。

The original uses だ, which is plain style.
So the difference is mostly level of politeness:

• だ = plain / casual / neutral written style
• です = polite

Japanese often uses short sentences for a clear, punchy explanation.

Here the flow is:

1. These features are present.
2. So, naturally, it’s convenient.

That makes the logic feel direct and easy to follow.

A longer one-sentence version is also possible, for example:

• 前かご、荷台、泥よけ、チェーンカバー、両立スタンドがあるので、便利なわけだ。

But the two-sentence version can sound more emphatic, almost like:

• Look at all these features. Of course it’s convenient.

So the split is a stylistic choice, not a grammatical problem.