senpai wo hirugohan ni sasou toki ha, teineina iikata wo sita hou ga ii.

Because 誘う normally takes the person being invited as its direct object.

• 人を誘う = to invite someone
• 先輩を誘う = to invite your senior

Then 昼ご飯に tells you what you are inviting them to.

So the pattern is:

• [person] を [event/occasion] に 誘う

Example:

• 友だちを映画に誘う = invite a friend to a movie
• 先輩を昼ご飯に誘う = invite a senior to lunch

So を is completely normal here.

Here, に marks the purpose or destination of the invitation.

So 昼ご飯に誘う means invite someone to lunch.

This に is not a location marker here. It is closer to to in English:

• 食事に誘う = invite someone to a meal
• デートに誘う = invite someone on a date
• 昼ご飯に誘う = invite someone to lunch

So 昼ご飯に is telling you what kind of outing or event the invitation is for.

Because とき is a noun, and the verb before it modifies it directly. In this kind of grammar, Japanese uses the plain form, not the polite -ます form.

So:

• 誘うとき = when inviting / when you invite
• not 誘いますとき

This is very common in Japanese. Before nouns like とき, 人, こと, 場所, etc., verbs usually appear in plain form.

Examples:

• 食べるとき = when eating
• 行くとき = when going
• 会ったとき = when I met / when meeting after having met

The difference is about timing.

• 誘うとき = when you are about to invite someone / when inviting someone
• 誘ったとき = when you invited someone / after inviting someone

In this sentence, the advice is about how to speak at the time you are making the invitation, so 誘うとき is the right choice.

If you said 誘ったとき, it would sound more like talking about what happened after the invitation was made.

The は makes when inviting a senior to lunch the topic of the sentence.

So 誘うときは is like:

• as for when you invite a senior to lunch...
• when inviting a senior to lunch...

It gives the sentence a general, advice-like feeling.

Without は, the sentence would still be possible, but ときは sounds more natural when giving a rule, tip, or general recommendation.

Because 丁寧 is a na-adjective.

Na-adjectives take な when they directly modify a noun:

• 丁寧な言い方 = a polite way of speaking
• 静かな場所 = a quiet place
• 便利なアプリ = a convenient app

So here:

• 丁寧 = polite
• 言い方 = way of saying something
• 丁寧な言い方 = polite wording / a polite way of saying something

言い方 means way of saying something, wording, or phrasing.

It comes from:

• 言う = to say
• 方 = way, manner

So 言い方 is not just the content of what you say, but how you phrase it.

That is why this sentence is not only about being polite in attitude, but also about choosing polite wording.

For example, these can express a similar idea:

• 言い方に気をつける = be careful about your wording
• 丁寧な言い方をする = use polite wording

Both are possible, but they focus on slightly different things.

• 丁寧に言ったほうがいい = you should say it politely
• 丁寧な言い方をしたほうがいい = you should use polite wording / a polite way of speaking

The version with 言い方 emphasizes the phrasing itself. It sounds like advice about choosing the right expression, not just having a polite attitude.

Also, 言い方をする is a common pattern in Japanese:

• 変な言い方をする = speak in a strange way
• やさしい言い方をする = use gentle wording
• 丁寧な言い方をする = use polite wording

This is a very common grammar pattern:

• V-たほうがいい = had better do / it would be better to do

So:

• したほうがいい = it would be better to do

Even though した looks like past tense, in this pattern it does not mean past time. It is just the form used in the grammar pattern.

Examples:

• 早く寝たほうがいい = You should go to bed early
• 聞いたほうがいい = You should ask
• 丁寧な言い方をしたほうがいい = You should use polite wording

This pattern usually gives advice or a recommendation, not a strict order.

Not necessarily. 先輩 means someone senior to you in a group such as:

• school
• work
• a club
• a team

A 先輩 is usually someone who joined earlier or has more experience/status in that setting. They may be older, but they do not have to be.

That is why politeness matters here: the sentence is about speaking appropriately to someone above you in the social hierarchy.

No. Japanese is normally written without spaces.

So this would usually appear as:

先輩を昼ご飯に誘うときは、丁寧な言い方をしたほうがいい。

The spaces are just there to make it easier for learners to see the parts of the sentence.