Проверь, не забыл ли ты проездной.

Проверь is the informal singular imperative of проверить — to check.

So Проверь means Check or Make sure.

Russian uses the perfective verb проверить here because the speaker wants one complete check. It is not about an ongoing process, but about doing the check once and finishing it.

Compare:

• Проверь = Check
• Проверяй = Keep checking / check regularly / do the checking

In this sentence, Проверь is the natural choice.

This is a very common question.

In Russian, after verbs like check, see, ask, make sure, you will often get a clause with не where English might say whether or that you haven’t...

So:

• Проверь, не забыл ли ты проездной.

literally feels like:

• Check whether you haven’t forgotten your pass.

But natural English is simply:

• Check whether you forgot your pass
• Make sure you didn’t forget your pass

The не does not mean the whole sentence is a straightforward negation. It is part of a very normal Russian pattern in this kind of subordinate clause.

Ли is a particle used to form an indirect yes/no question. It often corresponds to English whether or if.

So:

• забыл ли ты = whether you forgot

This is not a direct question like:

• Ты забыл проездной? = Did you forget your pass?

Instead, it is an embedded question inside a larger sentence:

• Проверь, не забыл ли ты проездной. = Check whether you forgot your pass.

Ли normally comes right after the word it relates to most closely, often the verb.

So the neutral pattern here is:

• забыл ли ты

This is the standard way to say whether you forgot.

Different placement can change the emphasis:

• Ты ли забыл...? would emphasize you
• Проездной ли ты забыл...? would emphasize the pass

Those are marked and unusual here. The normal order is the one in your sentence.

Russian uses the past tense here because the possible forgetting would already have happened before the moment of checking.

So the logic is:

• First, maybe you forgot the pass.
• Now, check whether that happened.

That is why Russian says забыл — literally forgot.

This often corresponds to English forgot or have forgotten, depending on context.

So:

• не забыл ли ты can mean whether you forgot or whether you’ve forgotten

English and Russian do not match perfectly here, but the Russian tense is completely normal.

Here проездной is in the accusative singular, because it is the direct object of забыл.

But проездной is a masculine inanimate noun, and masculine inanimate nouns have the same form in the nominative and accusative singular.

So:

• nominative: проездной
• accusative: проездной

That is why it looks unchanged.

Проездной is a very common colloquial noun meaning a travel pass, transit pass, or transport pass.

Historically, it comes from expressions like:

• проездной билет = travel pass / season ticket

In everyday Russian, people often shorten this to just:

• проездной

So in this sentence it means something like:

• your transit pass
• your travel card
• your bus/metro pass

The exact English translation depends on the country and transport system.

Russian often leaves out possessive words when ownership is obvious from context.

Here, if someone says:

• не забыл ли ты проездной

it is naturally understood as:

• whether you forgot your pass

Adding твой or свой is possible, but not necessary.

For example:

• не забыл ли ты свой проездной = also correct
• не забыл ли ты твой проездной = possible in some contexts, but свой is usually better when the possessor is the subject

Russian often sounds more natural than English when it does not repeat possession unnecessarily.

Yes, Russian often omits subject pronouns, because the verb form already gives person and number.

So in some contexts you could hear:

• Проверь, не забыл ли проездной.

However, ты is very natural here and makes the sentence clearer and more complete, especially in a subordinate clause with ли.

So:

• не забыл ли ты проездной = neutral and clear
• не забыл ли проездной = more elliptical, more conversational, and more context-dependent

Including ты is a safe and natural choice.

Because the sentence has:

1. a main clause: Проверь
2. a subordinate clause: не забыл ли ты проездной

Russian separates those with a comma:

• Проверь, не забыл ли ты проездной.

This is standard punctuation when an imperative like check is followed by an embedded clause such as whether you forgot...

Yes. A few natural alternatives are:

• Проверь, не забыл ли ты свой проездной.
  Same meaning, with свой added.

• Посмотри, не забыл ли ты проездной.
  More like See if you forgot your pass.

• Убедись, что ты не забыл проездной.
  Make sure you didn’t forget your pass.

• Ты проездной не забыл?
  A direct, conversational question: You didn’t forget your pass, did you?

Your original sentence is completely natural and neutral.