watasi ha eki de hitori de matte iru tomodati ni hanasikakemasita.

A helpful chunking is:

• 私は
• 駅で
• 一人で
• 待っている
• 友達に
• 話しかけました

The key structure is:

• 駅で一人で待っている友達 = the friend who was waiting alone at the station
• 友達に話しかけました = spoke to / addressed the friend

So the long phrase before 友達 is all describing 友達.

Because Japanese puts a modifying clause before the noun it modifies.

So:

• 待っている友達 = the friend who is/was waiting

English uses words like who or that. Japanese does not. It simply places the whole description in front of the noun.

When a verb modifies a noun, Japanese normally uses the plain form, not the polite form.

So:

• 待っている友達 is correct
• 待っています友達 is not natural Japanese

Polite forms like 待っています are used when the verb is the main predicate of the sentence, not when it is directly attached to a noun as a modifier.

Because 待っている describes what the friend was doing at the time you spoke to them.

In Japanese, a non-past progressive form inside a noun-modifying clause often corresponds to English was -ing in past narration.

So here:

• 待っている友達に話しかけました naturally means I spoke to a friend who was waiting

Japanese is focusing on the friend as being in the state of waiting at that moment.

Yes, 待っていた友達 is also possible.

Very roughly:

• 待っている友達 focuses on the friend as the one who is in the middle of waiting
• 待っていた友達 looks back more directly on the waiting as a past event/state

In many contexts, both can be translated similarly in English. The original sentence is perfectly natural.

They do different jobs.

• 駅で: で marks the place where an action happens
  
  • at the station
• 一人で: で marks manner/state
  
  • alone, by oneself

So the two で are not the same kind of meaning, even though they look identical.

Most naturally, it describes the friend.

That is because 一人で is inside the phrase modifying 友達:

• 駅で一人で待っている友達
• the friend who was waiting alone at the station

So the usual reading is that the friend was alone.

If you wanted to say I was alone, you would normally phrase it differently.

Because で marks the location where the action takes place.

Here, the action is waiting, so:

• 駅で待っている = waiting at the station

By contrast, に often marks:

• destination
• existence/location with verbs like いる or ある
• a point in time

So with 待つ, 駅で is the natural choice for the place where the waiting happens.

Because 話しかける takes the person you speak to with に.

So:

• 友達に話しかけました = I spoke to / addressed my friend

This is just the pattern of the verb:

• X に 話しかける = speak to X

It is not treated like a direct object with を.

Yes, there can be.

Japanese often leaves out information that is understood from context or not important.

For example:

• 友達を待っている = waiting for a friend
• 電車を待っている = waiting for a train

But in your sentence, the speaker only needs to identify the person as a friend who was waiting. What that friend was waiting for is not necessary, so it is omitted.

は marks the topic.

So 私は is like saying:

• as for me
• or more simply in English, just I

It sets up the sentence from the speaker’s point of view.

If you used が instead, the nuance would shift more toward I am the one who..., which is a different kind of focus.

Yes, very often.

If the context already makes it obvious that the speaker is the one who spoke to the friend, Japanese would naturally omit 私 and simply say:

• 駅で一人で待っている友達に話しかけました。

Japanese frequently omits subjects and topics when they are clear from context.

Grammatically, it could be either, because Japanese nouns usually do not show number by themselves.

So 友達 can mean:

• friend
• friends

In this sentence, the most natural reading is usually a friend, but context is what really decides.

In this sentence, 駅で most naturally belongs to the modifier of 友達:

• 駅で一人で待っている友達
• the friend who was waiting alone at the station

So it describes where the friend was waiting.

That said, in real life, if you spoke to that friend while they were waiting there, the place of waiting and the place of speaking are probably the same anyway. But grammatically, 駅で is attached to the description of 友達.