Jag ringde henne i går, men hon svarade inte.

Because hon is the subject form, while henne is the object form.

In this sentence, jag is the subject doing the action, and henne is the person receiving the action.

Compare:

• Hon ringde mig. = She called me.
• Jag ringde henne. = I called her.

So this works like English she / her:

• hon = she
• henne = her

They are both in the simple past (also called the preterite).

• ringa → ringde
• svara → svarade

So the past meaning is built directly into the verb. Swedish often does this where English also uses a simple past form, but Swedish does not need an extra helping verb.

A useful thing to notice is that different verbs form the past in different ways, so you have to learn each verb pattern:

• svara is a very regular -ade past verb
• ringa has the past form ringde

Because Swedish normally does not use a dummy verb like English do/did for negatives.

English:

• She did not answer.

Swedish:

• Hon svarade inte.

The verb svarade already shows the past tense, and inte makes the clause negative.

The same thing happens in questions:

• English: Did she answer?
• Swedish: Svarade hon?

So Swedish usually expresses this with:

• the finite verb itself
• plus word order
• plus inte for negation

In a normal main clause, Swedish usually places inte after the finite verb.

So:

• hon svarade inte
• jag ringde inte
• vi kom inte

That is the usual pattern.

A very useful contrast:

• Main clause: Hon svarade inte.
• Subordinate clause: ... att hon inte svarade.

So in subordinate clauses, inte usually comes before the finite verb, but in main clauses it usually comes after it.

i går is the standard expression for yesterday.

Even though it looks like more than one word, learners should mostly treat it as a fixed time expression.

Examples:

• Jag arbetade i går.
• I går arbetade jag.

Both are natural. So i går behaves like an adverbial of time and can move around in the sentence.

Yes, absolutely:

I går ringde jag henne, men hon svarade inte.

This is very common.

But when you move i går to the front, Swedish keeps the finite verb in second position. That is why it becomes:

• I går ringde jag henne

and not:

• I går jag ringde henne

This is a very important Swedish word-order rule, often called V2 (verb second).

Because ringa takes a direct object.

So Swedish says:

• ringa någon = call someone

That means jag ringde henne is the normal structure.

You do not need a preposition there.

Compare:

• Jag ringde henne.
• Jag pratade med henne.

With prata, you need med. With ringa, you do not.

Because Swedish often leaves things unstated when the context makes them obvious.

After Jag ringde henne, the most natural meaning of men hon svarade inte is that she did not pick up or did not answer the call.

So Swedish does not always need an explicit object like:

• telefonen
• mitt samtal

If you want to be more explicit, you could say:

• hon svarade inte i telefon
• hon svarade inte när jag ringde

But the original sentence is completely natural as it stands.

It is very common and very natural here, because it separates two full clauses:

• Jag ringde henne i går
• men hon svarade inte

In Swedish, comma use can be a little more flexible than many English learners expect, but with men, a comma is often used when the two parts are complete clauses.

So this sentence looks perfectly normal with the comma.

In careful speech, jag is often pronounced something like yahg, but in everyday spoken Swedish it is very often reduced to something closer to ya.

So in natural speech, the beginning may sound more like:

• Ya ringde henne i går...

This reduction is extremely common, so it is good to recognize it when listening, even if you pronounce the fuller form yourself at first.