Learn Hardcore Japanese: konbini de koohii wo kau tame ni narabimasu. - I line up at the convenience store to buy coffee.

With verbs like 並ぶ (to line up / stand in line), you’re not “going to” the place as a destination in the grammar sense; you are performing the action there, so で is natural.

コンビニに並びます is generally unnatural for this meaning and is not how natives would say “line up at the convenience store.”

Why is コーヒー marked with を?

を marks the direct object of a verb — the thing the action is done to.

Verb: 買う = to buy
What is being bought? → コーヒー

Therefore:

コーヒーを買う = to buy coffee

So in the whole sentence:

コーヒーを買う = (in order) to buy coffee
コーヒーを is simply the object of 買う inside the 買うために phrase.

What does 買うために mean, and what kind of grammar pattern is this?

買うために is a standard purpose clause:

[dictionary-form verb] + ために = in order to [verb] / for the purpose of [verb]

So:

買う (dictionary form of 買います) = to buy
買うために = in order to buy

In your full sentence:

コーヒーを買うために並びます。
→ I line up in order to buy coffee.

This pattern is very common for expressing clear, intentional purpose:

日本語を勉強するために、日本に来ました。
→ I came to Japan in order to study Japanese.
健康を守るために、運動します。
→ I exercise in order to protect my health.

Why is it 買うために (dictionary form) and not 買いますために or 買いするために?

With ために expressing purpose, you must use the dictionary form of the verb:

買うために ✅
買いますために ❌
買いするために ❌

Basic rule:

Verb (dictionary form) + ために = in order to [verb]

Examples:

見るために (in order to see)
行くために (in order to go)
勉強するために (in order to study)

The polite ます-form (買います, 行きます) cannot be placed directly before ために in this usage.

Can I drop に and just say 買うため並びます?

In modern everyday Japanese, you normally say ために, not bare ため, when you mean “in order to”.

買うために並びます。 ✅ natural
買うため並びます。
- Grammatically not impossible, but sounds stiff, literary, or old-fashioned, and is not what you should imitate as a learner.

So: keep the に with this purpose expression: [verb-dictionary] + ために.

Could the word order be コーヒーを買うために、コンビニで並びます。? Is that more natural?

Yes, that word order is very natural, and many speakers might actually prefer it:

コーヒーを買うために、コンビニで並びます。
→ I line up at the convenience store in order to buy coffee.

Japanese word order is fairly flexible as long as:

Verbs go at the end, and
Particles stay attached to the words they mark.

Your original sentence:

コンビニでコーヒーを買うために並びます。

also works, but putting コーヒーを買うために before コンビニで can sometimes feel a bit clearer because it introduces the purpose first, then the location of the action.

Where is the subject “I” in this sentence? Why isn’t there a word like 私?

Japanese often omits the subject (and even the object) when it is clear from context.

コンビニでコーヒーを買うために並びます。

Literally is more like:

“At the convenience store, (someone) lines up in order to buy coffee.”

In a natural context (you talking about yourself), that someone is “I”, so the English translation adds I even though Japanese leaves it out.

You can say:

私はコンビニでコーヒーを買うために並びます。

but adding 私 every time often sounds unnatural or overly explicit in Japanese unless you need to contrast or clarify who the subject is.

What is the nuance and form of 並びます? What is the dictionary form?

並びます is the polite ます-form of the verb 並ぶ.

並ぶ (dictionary form) = to line up / stand in line / form a line
並びます = polite present/future form

Nuance:

Here it means to stand in line / queue up (intransitive).
- 人が店の前に並んでいます。 = People are lined up in front of the store.

Note the related transitive verb:

並べる = to line something up / to arrange in a line
- 本を棚に並べます。 = I line up the books on the shelf.

What tense is 並びます? Does it mean “I am lining up” or “I will line up” or “I usually line up”?

The ます-form (here: 並びます) is used for non-past, which covers:

Present habitual:
- I (usually) line up / I line up.
Future:
- I will line up.
Sometimes general statements.

Which it is depends on context:

Talking about your routine:
→ “I usually line up at the convenience store to buy coffee.”
Talking about what you’re about to do:
→ “I’m going to line up at the convenience store to buy coffee.”

Japanese doesn’t make a strict grammatical distinction between present and future the way English does; context does the work.

Could I say コーヒーを買いにコンビニで並びます instead of コーヒーを買うために並びます? What’s the difference?

You can say:

コーヒーを買いにコンビニで並びます。 ✅

Here 買いに is:

ます-stem of 買う (買い) + に
This also means “to (go/come) in order to buy”.

Rough nuance difference:

買うために
- Slightly more formal / explicit “for the purpose of buying.”
- Neutral and works well in writing or more careful speech.
買いに
- Very common and a bit more casual / conversational.
- Often used with motion verbs:
  - コーヒーを買いにコンビニへ行きます。
    → I go to the convenience store to buy coffee.

In your specific sentence, both are acceptable; 買いに feels just a bit more everyday-colloquial.

What exactly is コンビニ? Why is it in katakana?

コンビニ is the shortened, everyday word for コンビニエンスストア:

From English “convenience store”.
Because it’s a foreign loanword, it’s written in katakana.

So:

コンビニ ≈ convenience store (7-Eleven, Lawson, FamilyMart, etc.)

Many loanwords in Japanese are shortened like this:

パーソナルコンピューター → パソコン (personal computer → PC)
リモートコントロール → リモコン (remote control)

Can ために be used after nouns as well, like コーヒーのために並びます? How is that different from コーヒーを買うために並びます?

Yes, ために can follow nouns:

[Noun] のために = for the sake of [noun] / for [noun]

So:

コーヒーのために並びます。
Literally: I line up for the sake of coffee / for coffee.

However, this sounds a bit vague or dramatic in isolation (like “I line up for the sake of coffee itself”). To express “in order to buy coffee”, Japanese normally prefers a verb before ために:

コーヒーを買うために並びます。 ✅ clear, natural
→ line up in order to buy coffee

Using noun + のために focuses on that noun as a goal or beneficiary (for my health, for my family, for the company, etc.):

家族のために働きます。 = I work for my family.
健康のために走ります。 = I run for my health.

Why isn’t there any comma in Japanese like “コンビニで、コーヒーを買うために、並びます”? Are commas important?

Japanese can use commas (、), but they are:

Less strict than in English, and
Often omitted in short, simple sentences.

You could write:

コンビニで、コーヒーを買うために、並びます。

but many native writers would drop some or all of those commas because:

The meaning is clear without them.
Overusing commas can look fussy in Japanese.

Commas are mainly used to:

Avoid misreading,
Show a slight pause,
Separate longer or nested clauses.

In this case, your original sentence is short enough that commas are optional.