eki ni tuitara, sugu tomodati ni renrakusimasu.

This is one of the most common questions in Japanese.

着いたら comes from:

• 着く = to arrive
• 着いた = arrived
• 着いたら = when/if (someone) arrives

The -たら form is made from the past plain form, but it does not necessarily mean past time here. In this pattern, it marks a condition or sequence:

• 駅に着いたら = when I arrive at the station / after I arrive at the station

So even though 着いた looks past, 着いたら often means when/once/if in relation to a future situation.

In this sentence, 〜たら is best understood as when or once.

• 駅に着いたら、すぐ友達に連絡します。 = When/Once I arrive at the station, I’ll contact my friend right away.

Because arriving at the station is expected to happen, if sounds less natural in English unless the situation is uncertain.

You can think of 〜たら here as showing:

1. first action: arrive at the station
2. next action: contact the friend

So it has a strong sense of after that happens.

The particle に marks the destination with verbs like 着く.

• 駅に着く = to arrive at the station
• 家に着く = to arrive home
• 空港に着く = to arrive at the airport

So in 駅に着いたら, the に shows the place reached.

With 着く, に is the standard particle for the destination.

This に is a different use of the same particle.

In 友達に連絡します:

• 友達 = friend
• に = marks the target/recipient
• 連絡します = contact

So:

• 友達に連絡します = I’ll contact my friend

Here, に marks the person the action is directed toward.

So the sentence has two different に particles:

• 駅に = to/at the station as the destination of arrival
• 友達に = to my friend as the recipient of the contact

Japanese often leaves out the subject when it is obvious from context.

So instead of saying:

• 私は駅に着いたら、すぐ友達に連絡します。

Japanese very naturally says:

• 駅に着いたら、すぐ友達に連絡します。

The listener can usually understand that the subject is I from context.

This omission is extremely common in Japanese and makes sentences sound more natural.

すぐ means immediately, right away, or soon.

It modifies the action 連絡します, so it appears before that part of the sentence:

• すぐ連絡します = I’ll contact (someone) right away
• すぐ友達に連絡します = I’ll contact my friend right away

Japanese adverbs often come before the verb or verb phrase they modify.

So the placement is very normal:

• 駅に着いたら、すぐ友達に連絡します。

Usually, no.

• 友達に連絡します = contact my friend

With 連絡する, the person you contact is typically marked by に.

と usually means with and suggests doing something together or interacting mutually, but it does not normally replace に here.

Natural patterns are:

• 友達に連絡する = contact a friend
• 会社に連絡する = contact the company
• 先生に連絡する = contact the teacher

So 友達に連絡します is the correct and natural expression.

Both are possible, but 連絡します is more common and natural in everyday Japanese.

There are two related patterns:

• 連絡する
• 連絡をする

In many cases, Japanese often drops を with certain noun + する combinations, especially very common ones.

So:

• 友達に連絡します = very natural
• 友達に連絡をします = also grammatical, but a bit heavier/formal in many contexts

Most learners will hear and use 連絡する more often.

They are similar, but not the same.

1. 着いたら

• when/once I arrive
• natural for a future sequence of actions
• very common in everyday speech

2. 着くと

• can mean when in a more general or automatic sense
• often used for things that regularly happen or are factual
• can sound less suitable for a personal planned action like this

3. 着けば

• means if I arrive / if I can arrive
• focuses more on the condition than the sequence
• often sounds less natural than 〜たら in this specific sentence

For this sentence, 着いたら is the most natural choice because it means: After I arrive, I’ll do the next action.

連絡します is in the polite non-past form.

• dictionary/plain form: 連絡する
• polite form: 連絡します

This makes the sentence polite and appropriate in many normal situations.

Compare:

• 駅に着いたら、すぐ友達に連絡する。
  casual/plain
• 駅に着いたら、すぐ友達に連絡します。
  polite

So the sentence mixes:

• a plain conditional clause: 着いたら
• a polite main verb: 連絡します

That combination is completely normal in Japanese.

The comma helps separate the conditional part from the main action:

• 駅に着いたら、 = when I arrive at the station,
• すぐ友達に連絡します。 = I’ll contact my friend right away.

It makes the sentence easier to read.

In Japanese, commas are somewhat flexible, so you may sometimes see the sentence written without one:

• 駅に着いたらすぐ友達に連絡します。

That is also fine. The comma is just there to show a natural pause.

駅 usually means station, and in many contexts English naturally adds the:

• 駅に着いたら = when I arrive at the station

Japanese often does not mark the/a the way English does. Whether it is the station, a station, or my station depends on context.

In a sentence like this, the station is the most natural English translation, but the Japanese itself simply says station.

In Japanese, 着く means to arrive and is used with a destination marked by に.

So:

• 駅に着く = arrive at the station

In English, we usually say arrive at for smaller places like stations, schools, or airports, and arrive in for cities or countries. Japanese does not make that same distinction in the same way here; it just uses に.

So the key pattern to learn is:

• place + に + 着く

Yes, Japanese word order is fairly flexible as long as the particles make the relationships clear.

The original is very natural:

• 駅に着いたら、すぐ友達に連絡します。

You could also say:

• 駅に着いたら、友達にすぐ連絡します。

This is also understandable and natural enough, though the original flows a bit better.

What usually stays the same is that the verb comes near the end, and the 〜たら clause comes before the main result/action.

So while some reordering is possible, the given sentence is a very standard and natural way to say it.