Я открыла банку мёда, но пачку печенья решила оставить на завтра.

Because the Russian past tense agrees with the gender of the subject in the singular.

• Я открыла = I opened said by a woman
• Я открыл = I opened said by a man

The same applies to решила:

• я решила = a female speaker
• я решил = a male speaker

So this sentence tells you the speaker is female.

Because both words are direct objects, so they are in the accusative case.

Their dictionary forms are:

• банка = jar/can
• пачка = pack/packet

These are feminine nouns ending in -а, and in the singular accusative they usually change -а to -у:

• банка → банку
• пачка → пачку

So:

• открыла банку = opened a jar
• оставить пачку = leave a pack

Because Russian often uses the pattern:

container/package noun + genitive

So:

• банка мёда = a jar of honey
• пачка печенья = a pack of cookies/biscuits

The second noun tells you what the container or package contains, and it usually goes in the genitive.

Other common examples:

• чашка чая = a cup of tea
• бутылка воды = a bottle of water
• кусок хлеба = a piece of bread

Here it is best understood as genitive singular, required by пачка.

The dictionary form is печенье. After a container word like пачка, Russian uses the genitive:

• пачка печенья

A tricky point: the form печенья can look like a plural form too, but in this sentence its role is determined by the grammar after пачка. So here it means of cookies/biscuits, not a nominative plural subject.

Because решила оставить means decided to leave.

That adds the idea of a decision:

• решила оставить на завтра = decided to leave it for tomorrow
• оставила на завтра = left it for tomorrow

So решила makes the action sound more deliberate: she made a choice not to open the cookies yet.

They are perfective because the sentence talks about single, complete actions or their result.

• открыла from открыть = opened, completed the action
• оставить = to leave/save for later, as one complete decision/action

Russian aspect matters a lot:

• perfective = a completed whole action
• imperfective = process, repetition, or ongoing action

Compare:

• Я открыла банку мёда = I opened the jar of honey
• Я открывала банку мёда = I was opening / used to open the jar of honey

And:

• решила оставить = decided to leave
• решила оставлять would usually sound wrong here unless you meant a repeated habit

It can, but it does not have to.

Russian often omits repeated subject pronouns when the subject is already clear:

• Я открыла банку мёда, но пачку печенья решила оставить на завтра.

This naturally means that the same person did both actions.

You could also say:

• Я открыла банку мёда, но я решила оставить пачку печенья на завтра.

That version is more explicit and can sound slightly more emphatic, but the shorter version is more natural in many contexts.

Because оставить на завтра is a very common Russian pattern meaning:

• leave/save/postpone for tomorrow

The preposition на shows that something is being set aside for a future time.

So:

• оставить на завтра = leave it for tomorrow

If you just said оставить завтра, it would not mean the same thing and would usually sound odd in this context.

Similar patterns:

• отложить на потом = put off until later
• перенести на понедельник = move/postpone to Monday

Russian word order is flexible, and different orders can shift the emphasis.

This sentence puts пачку печенья earlier to highlight the contrast:

• I opened the jar of honey, but the pack of cookies I decided to leave for tomorrow.

So the structure helps balance the two objects:

• банку мёда ... but ...
• пачку печенья ...

A more neutral order would also be possible:

• Я открыла банку мёда, но решила оставить пачку печенья на завтра.

That is also correct, just slightly different in focus.

Because но means but, and it joins two clauses.

Russian normally uses a comma before coordinating conjunctions like но when they connect clauses:

• Я открыла банку мёда, но ...
• I opened the jar of honey, but ...

So the comma here is standard punctuation.

The correct pronunciation here is with ё:

• мёд = honey
• мёда = of honey

In many Russian texts, especially less formal ones, ё is often written as е, so you may see меда in writing. But the word is still pronounced myóda, not méda.

For learners, it is helpful to remember the dictionary form with ё:

• мёд
• not мед in pronunciation

So even if the dots are omitted in real texts, the underlying word is still мёд.