Я оставил клей, ножницы и линейку на столе, потому что вечером собираюсь доделать проект.

This is a question about aspect.

• оставил is perfective
• оставлял is imperfective

Here, оставил presents the action as a completed whole: the speaker left the glue, scissors, and ruler on the table, and that action is now done.

If you used оставлял, it would sound more like:

• a repeated or habitual action
• a process
• or background information, depending on context

In this sentence, the speaker means I left them there as a completed action, so оставил is the natural choice.

Because these words are the direct objects of оставил, so they need the accusative case.

Here is what happens:

• клей → клей
  masculine inanimate singular; accusative = nominative

• ножницы → ножницы
  plural-only inanimate noun; accusative = nominative

• линейка → линейку
  feminine singular noun ending in -а, so accusative changes -а to -у

So the sentence mixes nouns whose accusative forms happen to look different or identical depending on gender and declension.

Because на can mean either:

• onto a surface, with movement toward it
• on a surface, with location

The case tells you which meaning is intended:

• на стол = onto the table → accusative
• на столе = on the table → prepositional

In this sentence, the items are already located there, so Russian uses на столе.

In Russian, a clause introduced by потому что is normally separated by a comma.

So:

• main clause: Я оставил клей, ножницы и линейку на столе
• subordinate clause: потому что вечером собираюсь доделать проект

Russian punctuation is generally stricter here than English. Even if English sometimes feels flexible with because, Russian normally wants the comma.

Because вечером is using the instrumental case in an adverbial time expression.

Russian often uses certain nouns in the instrumental to mean things like:

• утром = in the morning
• днём = during the day
• вечером = in the evening
• ночью = at night

So вечером here simply means in the evening.
You do not say в вечером.

You can say в этот вечер for this evening in some contexts, but the simple general expression is just вечером.

Собираюсь means something like I am planning to, I intend to, or I’m getting ready to.

The pattern is:

• собираться + infinitive

So:

• собираюсь доделать = I’m planning to finish off / complete

This does not simply state the future. It adds the idea of intention or plan.

Compare:

• вечером доделаю проект = I’ll finish the project this evening
• вечером собираюсь доделать проект = I’m planning to finish the project this evening

The second one sounds a bit softer and more about intention.

Доделать means to finish doing something that is already partly done.

The prefix до- often adds the idea of completing what remains.

So:

• сделать проект = do / make the project
• закончить проект = finish the project
• доделать проект = finish off the project, complete the remaining part

In this sentence, доделать suggests the project is already underway, and the speaker plans to complete the rest later in the evening.

It is in the accusative.

The verb доделать takes a direct object, so проект must be accusative. But for many masculine inanimate singular nouns, the accusative looks exactly like the nominative.

So:

• nominative: проект
• accusative: проект

The case is there grammatically, even though the form does not change.

Russian often omits a subject pronoun when it is already clear from context.

The first clause begins with Я оставил..., so the subject is established.
Then собираюсь is a 1st person singular verb form, which already tells you the subject is I.

So Russian does not need to repeat я, although it could if the speaker wanted emphasis:

• ..., потому что я вечером собираюсь доделать проект.

That version is possible, but less neutral.

Yes, Russian word order is fairly flexible.

This version is neutral and natural:

• вечером собираюсь доделать проект

But you could also hear:

• собираюсь вечером доделать проект
• проект собираюсь вечером доделать

The basic meaning stays similar, but the focus changes.

For example:

• вечером first can highlight the time
• проект first can highlight the thing being finished

Russian uses word order more for emphasis, information structure, and style than English does.

Yes. Ножницы is a plural-only noun in Russian, just like scissors in English.

That means it does not normally have a singular form in everyday usage. So you say:

• эти ножницы = these scissors
• ножницы лежат на столе = the scissors are lying on the table

Even though it refers to one object, grammatically it behaves as plural.

Here оставил means left in the sense of left something somewhere intentionally or as a result of placing it there and not taking it away.

It is broader than English left in some contexts. Depending on situation, оставить can mean:

• leave behind
• leave somewhere
• keep something somewhere
• abandon
• let remain

In this sentence, the idea is:

• the speaker left those items on the table
• because they plan to continue working later

So left on the table is the best natural understanding.

Yes. A very natural alternative would be:

• ..., потому что вечером доделаю проект.

That means:

• ..., because I’ll finish the project this evening.

The difference is:

• доделаю = straightforward future fact or intention
• собираюсь доделать = plan/intention, often a little less direct and slightly more conversational

Both are correct, but собираюсь emphasizes that this is the speaker’s plan.