Если ты обиделся на подругу, лучше сказать ей об этом спокойно.

Because обиделся is the reflexive form of the verb: обидеться.

• обидеть = to offend someone
• обидеться = to get offended / to take offense

So:

• Ты обидел подругу = You offended your female friend
• Ты обиделся на подругу = You got offended at your female friend

The -ся changes the meaning a lot here. Without it, the sentence would mean the opposite kind of action.

Because the sentence means something like If you have gotten offended or If you got upset/offended. Russian often uses the past tense in если clauses when talking about a situation that may already have happened.

So the structure is:

• Если ты обиделся... = If you got offended / if you have taken offense...
• лучше сказать... = it’s better to say...

The advice applies after that situation has occurred.

Because the standard pattern is:

• обидеться на кого? на что?

So Russian says:

• обидеться на друга
• обидеться на учителя
• обидеться на шутку

That is just the normal government of this verb. After на, you use the accusative case here, which is why it is подругу.

Because на after обидеться на requires the accusative case.

The base form is:

• подруга = female friend

Accusative singular:

• подругу

So:

• на подругу = at your female friend / because of your female friend

This is not the person receiving speech in the sentence. It is the person toward whom the feeling of offense is directed.

Подруга literally means female friend. In many contexts, that is the safest meaning.

However, depending on context, it can sometimes also be understood as:

• a close female friend
• a girlfriend/partner in informal speech

In this sentence, without more context, a learner should understand it primarily as female friend. If Russian wants to be clearly romantic, people often use other wording too, depending on style and situation.

Because лучше сказать is a very common impersonal advice pattern in Russian.

It literally works like:

• Лучше сказать ей об этом спокойно = It’s better to tell her about it calmly

Even though ты is not repeated, it is understood from the context. Russian often leaves out pronouns when they are clear.

You could say:

• Лучше скажи ей об этом спокойно

That sounds more direct: You’d better tell her about it calmly.

The version in your sentence is a bit more neutral and general in tone.

Because сказать is perfective and refers to a single, completed act of saying something.

Here the idea is:

• if this situation happens, it is better to tell her about it

That suits сказать well.

Compare:

• сказать = to say / tell once, as a completed action
• говорить = to speak / to be saying / to say in a general or repeated sense

So:

• лучше сказать ей об этом = better to tell her about it
• лучше говорить спокойно = better to speak calmly

If you used говорить in this sentence, the focus would shift toward the manner or ongoing conversation, not the single act of bringing it up.

Because ей is the dative case of она and means to her.

The verb pattern is:

• сказать кому? что?
• tell someone something

So:

• сказать ей = tell her
• сказать другу = tell a friend
• сказать маме = tell mom

In the sentence:

• ей = to her
• об этом = about it / about that

Об этом means about it or about that.

The preposition is basically о = about, concerning. But before certain sounds, Russian uses a different form for pronunciation:

• о before many consonants
• об before vowels
• sometimes обо in a few fixed situations

Since этом begins with a vowel sound, Russian says:

• об этом

not о этом

Also, after о / об, Russian uses the prepositional case, so:

• это → этом

It refers to the fact that you were offended / your feelings were hurt.

So the sentence means that instead of bottling it up or reacting badly, it is better to tell her calmly about that, meaning:

• about your hurt feelings
• about the fact that you were offended
• about what upset you

Russian often uses об этом as a compact way to refer back to the whole previous situation.

Because it describes how you should say it.

• спокойный = calm (adjective)
• спокойно = calmly / in a calm way (adverb)

Here it modifies сказать:

• сказать спокойно = to say/tell calmly

So the sentence is advising not only what to do, but also how to do it.

Yes. Russian word order is fairly flexible, although different orders can change emphasis.

Your sentence:

• Если ты обиделся на подругу, лучше сказать ей об этом спокойно.

Other possible versions:

• Если ты обиделся на подругу, лучше спокойно сказать ей об этом.
• Если ты на подругу обиделся, лучше сказать ей об этом спокойно.

These all mean roughly the same thing, but the emphasis shifts slightly:

• спокойно before сказать can emphasize the calm manner a bit earlier
• на подругу earlier can emphasize who you are offended at

The original version sounds natural and neutral.

Yes. In the past tense, Russian verbs agree with gender in the singular.

So:

• to a man: Если ты обиделся на подругу...
• to a woman: Если ты обиделась на подругу...

Other parts of the sentence stay the same:

• лучше сказать ей об этом спокойно

If the sentence used вы instead of ты, then the past tense form would be plural:

• Если вы обиделись на подругу...

Because Если ты обиделся на подругу is a subordinate clause introduced by если.

Russian normally separates this kind of clause with a comma:

• Если..., ...

So the structure is:

• Если ты обиделся на подругу, = If you got offended at your female friend,
• лучше сказать ей об этом спокойно. = it’s better to tell her about it calmly.

This comma is required in standard writing.

Yes, but the nuance changes.

• обиделся на подругу = got offended / took offense
• обижен на подругу = is offended at / feels hurt by

So:

• Если ты обиделся на подругу... focuses more on the event of becoming offended
• Если ты обижен на подругу... focuses more on the current state

Both can work, but the original sentence sounds very natural when talking about a reaction that happened and now needs to be handled well.