watasi ha ryokousaki kara tomodati ni hagaki wo dasimasita.

は marks 私 as the topic of the sentence.

So 私は means something like as for me or I in this context.

It does not always mean the grammatical subject in the same way English I does. It tells the listener, we are talking about me now.

In this sentence, 私は sets up the speaker as the person who did the action:

• 私は = as for me
• 出しました = sent/mailed

So the whole sentence is talking about what I did.

旅行先 means the place where one is traveling or travel destination.

It is made from:

• 旅行 = travel, trip
• 先 = destination, place one goes to, the place ahead

So 旅行先から means from the place I was traveling or from my travel destination.

This is more specific than just 旅行から, which would mean from the trip in a broader sense.

から means from.

Here it shows the starting point or origin of the postcard:

• 旅行先から = from the travel destination

So the postcard was sent from that place.

This is the same から you see in other sentences like:

• 日本から来ました = I came from Japan.
• 9時から始まります = It starts from 9 o’clock.

In this sentence, it marks where the postcard was sent from.

に marks the recipient of the action.

So:

• 友達に = to a friend / to my friend

In a sentence about giving, sending, or saying something, に often marks the person who receives it.

Examples:

• 先生に手紙を出しました = I sent a letter to the teacher.
• 友達にプレゼントをあげました = I gave a present to my friend.

Here, 友達に tells us who received the postcard.

を marks the direct object, meaning the thing directly affected by the action.

So:

• はがきを = the postcard

The verb 出しました is the action, and はがき is what was sent.

This is very common:

• 本を読みました = I read a book.
• ご飯を食べました = I ate a meal.
• はがきを出しました = I sent a postcard.

Yes, 出す has a broad basic meaning like to put out, to take out, or to send out.

In Japanese, はがきを出す is a very natural expression meaning to mail/send a postcard.

The same pattern is used with:

• 手紙を出す = to mail/send a letter
• メールを出す can also be used, though メールを送る is often more common

So in this sentence, 出しました does not mean physically took out a postcard. It means sent/mailed it.

Yes, you often can, but the nuance is a little different.

• はがきを出しました = mailed/sent a postcard
• はがきを送りました = sent a postcard

Both are understandable, but はがきを出す is especially common for letters and postcards. It has the feeling of posting/mailing something.

送る is a more general verb for send.

So:

• 友達にはがきを出しました sounds very natural for mailing a postcard
• 友達にはがきを送りました is also fine, but slightly less like the fixed expression learners often meet first

Because Japanese normally puts the verb at the end of the sentence.

The basic pattern is often:

• topic / subject
• time / place / recipient / object
• verb

So this sentence follows a very normal Japanese word order:

• 私は = topic
• 旅行先から = from the travel destination
• 友達に = to a friend
• はがきを = postcard
• 出しました = sent

English usually puts the verb earlier, but Japanese saves it for the end.

Yes, Japanese word order is somewhat flexible because particles show each word’s role.

For example, these can all be natural depending on emphasis:

• 私は旅行先から友達にはがきを出しました。
• 私は友達に旅行先からはがきを出しました。
• 旅行先から私は友達にはがきを出しました。

However, the verb usually stays at the end, and some orders sound more natural than others depending on context.

The version you were given is a very clear, standard order.

Yes. Very often it would be omitted if the subject is already clear from context.

Japanese frequently leaves out information that both speaker and listener already understand.

So in real conversation, someone might simply say:

• 旅行先から友達にはがきを出しました。

This still means I sent a postcard to a friend from my travel destination, if it is obvious that the speaker is talking about themselves.

Including 私 makes it more explicit.

出しました is the polite past form of 出す.

Breakdown:

• dictionary form: 出す = to send / to put out
• polite non-past: 出します
• polite past: 出しました

So 出しました means sent or mailed in a polite style.

If you made it plain/casual, it would be:

• 出した

So:

• 私は旅行先から友達にはがきを出しました。 = polite
• 私は旅行先から友達にはがきを出した。 = casual/plain

Normally, Japanese is not written with spaces, so it would usually appear as:

私は旅行先から友達にはがきを出しました。

A natural reading is:

わたし は りょこうさき から ともだち に はがき を だしました。

Word readings:

• 私 = わたし
• 旅行先 = りょこうさき
• 友達 = ともだち
• 出しました = だしました

The spaces in learning materials are just there to make the parts easier to see.