Перед прогулкой я надеваю на собаку ошейник и беру поводок.

Because the preposition перед normally requires the instrumental case when it means before.

So:

• прогулка = walk
• перед прогулкой = before the walk

For a feminine noun ending in -а, the instrumental singular often ends in -ой:

• прогулка → прогулкой

So this is a case-governed form, not the dictionary form.

Here перед means before, as in before going for a walk.

But перед and до are not always interchangeable:

• перед прогулкой = before the walk, often right before it
• до прогулки = before the walk, but more abstractly or as a time limit

In this sentence, перед прогулкой sounds very natural for a routine that happens immediately before the walk.

This is a very common learner question.

In standard Russian:

• надевать / надеть = to put something on
• одевать / одеть = to dress someone

So:

• надеваю ошейник = I put a collar on
• одеваю ребёнка = I dress a child

Since a collar is the thing being put on, Russian uses надевать.

A useful memory trick:

• надеть что-то
• одеть кого-то

Because the pattern is:

• надевать что? на кого?
• to put something on someone/something

So in this sentence:

• ошейник = what is being put on
• на собаку = on the dog

Russian expresses this as put a collar onto the dog, so на is the natural preposition here.

Because на собаку uses the accusative case, which is used here to show direction/result of placement: the collar is being put onto the dog.

Compare:

• на собаку = onto the dog
• на собаке = on the dog

In this sentence, the action is not describing where the collar already is; it describes putting it there. That is why на + accusative is used.

Because after на in this construction, the noun is in the accusative case.

• dictionary form: собака
• accusative singular: собаку

This is a regular change for feminine nouns ending in -а.

Because ошейник is the direct object of надеваю, and it is an inanimate masculine noun.

For many inanimate masculine nouns, the accusative singular looks exactly like the nominative singular:

• nominative: ошейник
• accusative: ошейник

So the form does change grammatically to accusative, but it just happens to look the same.

In Russian, the present tense of an imperfective verb is often used for:

• habitual actions
• repeated routines
• general facts

So this sentence means something like:

• Before a walk, I put a collar on the dog and take the leash
• or Before going for a walk, I usually put a collar on the dog and take the leash

It is not necessarily happening right now. It can describe a regular habit.

Because the sentence describes a normal repeated routine, not a single completed event.

• надевать / надеваю = imperfective
• брать / беру = imperfective

If you wanted to talk about one completed occasion, you would more likely use perfective verbs, for example:

• надену
• возьму

But for what I do before a walk, imperfective is the natural choice.

Yes, very often.

Russian verb endings usually make the subject clear:

• надеваю already means I put on
• беру already means I take

So Перед прогулкой надеваю на собаку ошейник и беру поводок is also possible.

Including я makes the sentence a little clearer, fuller, or slightly more explicit, but it is not strictly necessary.

Yes, Russian word order is flexible.

This version is a neutral, natural order:

• Перед прогулкой я надеваю на собаку ошейник и беру поводок.

But other orders are possible depending on emphasis, for example:

• Я перед прогулкой надеваю на собаку ошейник и беру поводок.
• Ошейник я надеваю на собаку перед прогулкой...

The original sentence sounds like a normal, straightforward statement.

Russian has no articles like a, an, or the.

So:

• ошейник can mean a collar or the collar
• поводок can mean a leash or the leash

The exact meaning comes from context. In this sentence, English would probably translate them as the collar and the leash, because they are specific familiar objects in the situation.

Both are direct objects, but they behave slightly differently in form.

• ошейник is the thing being put on the dog
• поводок is the thing being taken

Both are masculine inanimate nouns, so in the accusative singular they look like the nominative:

• ошейник
• поводок

That is why neither one changes form here.

Yes, absolutely.

• беру поводок = I take the leash
• беру с собой поводок = I take the leash with me

Adding с собой makes the idea of taking it along more explicit. Without it, the original sentence is still completely natural and clear.