Я положу дождевик в сумку на всякий случай.

Положу is the 1st person singular form of the perfective verb положить.

In Russian, perfective verbs do not have a true present tense. Their “present-looking” forms actually refer to the future. So:

• я положу = I will put
• он положит = he will put

This is different from the imperfective pair:

• класть = to put, to be putting
• я кладу = I am putting / I put

Here, the speaker means one completed action—putting the raincoat into the bag—so положу is the natural choice.

Because it is a masculine inanimate noun in the accusative singular, and for masculine inanimate nouns, the accusative looks the same as the nominative.

So:

• nominative: дождевик
• accusative: дождевик

It is the direct object of положу—the thing being put.

Compare with an animate noun:

• Я вижу брата = I see my brother

There, the accusative changes. But with an inanimate noun like дождевик, it stays the same.

Because Russian uses different cases after в depending on whether you mean:

• movement into somewhere → accusative
• location inside somewhere → prepositional

So:

• в сумку = into the bag
• в сумке = in the bag

In this sentence, the raincoat is being moved into the bag, so в сумку is correct.

Compare:

• Я положу дождевик в сумку. = I’ll put the raincoat into the bag.
• Дождевик лежит в сумке. = The raincoat is in the bag.

Сумку is accusative singular of сумка.

For many feminine nouns ending in -а, the accusative singular changes to -у:

• сумка → сумку
• книга → книгу
• машина → машину

So в сумку literally means into a/the bag.

На всякий случай is a very common fixed expression meaning just in case.

Literally, it is something like for any case / for any eventuality, but in real usage you should learn it as a whole phrase.

Grammar inside the phrase:

• на
  • accusative
• всякий agrees with случай
• случай is masculine singular inanimate, so the accusative looks like the nominative

That is why you get:

• на всякий случай

not на всякого случая or anything like that.

Yes. Russian often drops subject pronouns when the verb ending already makes the subject clear.

So both are possible:

• Я положу дождевик в сумку на всякий случай.
• Положу дождевик в сумку на всякий случай.

The ending -у in положу already tells you it means I will put.

Including Я can sound slightly more explicit, contrastive, or simply natural in context. Omitting it is also very normal.

No. Russian word order is fairly flexible, and changing it usually changes emphasis, not the basic meaning.

The original order is natural and neutral:

• Я положу дождевик в сумку на всякий случай.

Other possible orders:

• На всякий случай я положу дождевик в сумку.
  Emphasis on just in case.

• Дождевик я положу в сумку на всякий случай.
  Emphasis on the raincoat.

• Я положу в сумку дождевик на всякий случай.
  Also possible, with slightly different focus.

So the original sentence is a normal, straightforward way to say it, but it is not the only possible order.

Because я положу is better for one complete action.

• положу = I will put
• буду класть = I will be putting / I will put repeatedly or over a period of time

In this sentence, the speaker means a single practical action: I’ll put the raincoat in the bag. Russian normally uses the perfective future for that:

• Я положу дождевик в сумку.

Using буду класть here would sound unusual unless you specifically meant an ongoing or repeated process.

Because Russian has no articles.

English needs words like a and the, but Russian usually leaves that information to context.

So:

• дождевик can mean a raincoat or the raincoat
• в сумку can mean into a bag or into the bag

If Russian wants to be more specific, it uses other tools, such as word order, context, or words like:

• этот = this
• мой = my
• тот = that

For example:

• Я положу этот дождевик в мою сумку. = I’ll put this raincoat in my bag.