hagaki no ura ni mizikai messeezi wo kakimasita.

の links two nouns together. In this sentence, はがき の 裏 means the back of the postcard.

A good way to think of it is:

• はがき = postcard
• 裏 = back / reverse side
• はがき の 裏 = the postcard’s back

So の is working a bit like of or ’s in English.

に marks the place where the action happens for verbs like 書く when you are writing on or in something.

So:

• はがき の 裏 に = on the back of the postcard

Even though English uses on, Japanese often uses に for the location where writing is placed.

を marks the direct object of the verb, meaning the thing that is being acted on.

Here:

• 短い メッセージ = a short message
• 短い メッセージ を 書きました = wrote a short message

So を shows that message is the thing being written.

In Japanese, adjectives usually come before the noun they describe, just like in English.

So:

• 短い メッセージ = short message

短い is an い-adjective, and it directly modifies メッセージ.

Japanese basic sentence order is often subject–object–verb, so the verb usually comes at the end.

This sentence is arranged like:

• はがき の 裏 に = on the back of the postcard
• 短い メッセージ を = a short message
• 書きました = wrote

So the final verb tells you what happened after the details have been set up.

書きました is the past polite form of 書く, so it means wrote.

Compare:

• 書きます = write / will write
• 書きました = wrote

So this sentence is talking about a completed action in the past.

The dictionary form is 書く, which means to write.

The form changes like this:

• dictionary form: 書く
• polite non-past: 書きます
• polite past: 書きました

This is a very common verb pattern in Japanese.

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

So this sentence could mean:

• I wrote a short message on the back of the postcard
• He/She wrote a short message on the back of the postcard

Without more context, Japanese does not force you to say who did it. The subject is simply omitted.

裏 means the back side or reverse side of something.

That makes it a natural choice for things like:

• paper
• cards
• postcards
• photos

So はがき の 裏 means the side opposite the front side of the postcard.

A word like 後ろ usually means behind or the rear, and it is used more for physical position rather than the reverse side of a flat object.

メッセージ is a loanword, borrowed from English message, so it is written in katakana.

Katakana is commonly used for:

• foreign loanwords
• some sound effects
• emphasis
• certain names

So メッセージ looks different from native Japanese words because of its origin.

Yes. Normal Japanese writing usually does not use spaces between words.

So it would normally be written as:

はがきの裏に短いメッセージを書きました。

Spaces are often added in beginner materials to make the sentence easier to read.

Yes. はがき can be written as 葉書.

However, it is very common to see it written in hiragana as はがき, especially in learner materials or everyday writing where the kanji may feel less necessary.

So both are possible:

• はがき
• 葉書