Я думаю об этом каждый день.

Because the verb думать in the meaning to think about normally uses the preposition о / об + the prepositional case.

• это is the dictionary form
• after о / об, it becomes этом

So:

• думать об этом = to think about this / it

Also, Russian uses об instead of о before certain vowel sounds for easier pronunciation. Since этом begins with э-, you say об этом, not о этом.

Этом is the prepositional case form of это.

Here is the pattern:

• nominative: это
• prepositional: об этом

This happens because о / об usually requires the prepositional case when it means about.

Думаю is the 1st person singular form of думать.

So it literally means:

• I think
• or in this sentence, more naturally, I think about

Its grammar is:

• infinitive: думать
• я думаю = I think / I am thinking

Russian present tense can cover both simple present and ongoing present depending on context.

Because думать is imperfective, and that fits a repeated or ongoing action.

In this sentence, каждый день means every day, so the action is habitual. That strongly favors the imperfective verb:

• думать = to think, to be thinking, to think repeatedly/habitually
• подумать = to think for a moment, to give something some thought, usually a more completed single action

So:

• Я думаю об этом каждый день = I think about it every day
• Я подумаю об этом = I’ll think about it

Yes, Russian often omits subject pronouns when they are obvious from the verb ending.

Since думаю already means I think, you could also say:

• Думаю об этом каждый день.

That still means I think about it every day.

Including Я can make the sentence:

• clearer
• slightly more explicit
• a bit more contrastive or personal, depending on context

So both are possible.

Каждый день is the normal Russian way to say every day.

Literally, it is:

• каждый = each / every
• день = day

Russian uses this expression very naturally as an adverbial time phrase.

Also, день here is in the form you would expect after каждый:

• каждый день = every day

This is a fixed, very common phrase.

In this expression, день is in the accusative singular, but because день is an inanimate masculine noun, its accusative form looks the same as the nominative:

• nominative: день
• accusative: день

So even though the form does not change visibly, grammatically it is functioning as part of a time expression meaning every day.

Yes. Russian word order is flexible, and the meaning stays basically the same, though the emphasis can shift.

Your sentence:

• Я думаю об этом каждый день.

Other possible versions:

• Каждый день я думаю об этом.
• Об этом я думаю каждый день.

Typical emphasis differences:

• Я думаю об этом каждый день. — neutral
• Каждый день я думаю об этом. — emphasizes every day
• Об этом я думаю каждый день. — emphasizes about this / about it

So word order in Russian often affects focus more than basic meaning.

Yes, but then it usually means something different.

Compare:

• Я думаю об этом. = I think about it.
• Я думаю, что он прав. = I think that he is right.

So:

• думать о / об чём-то = to think about something
• думать, что... = to think that...

You usually cannot say Я думаю это to mean I think about it. For that meaning, you need о / об.

Yes. The preposition has several forms:

• о
• об
• обо

These are mainly chosen for pronunciation.

For example:

• о книге = about a book
• об этом = about this
• обо мне = about me

You do not need to memorize every detail at once, but it is useful to know:

• об is commonly used before words beginning with a vowel sound
• обо appears in a smaller number of set combinations

So об этом is the normal standard form.

A simple learner-friendly pronunciation guide would be:

• Я — ya
• ду́маю — DOO-ma-yu
• об э́том — ob EH-tam
• ка́ждый день — KAZH-diy dyen’

Stress:

• я ду́маю об э́том ка́ждый день

A few useful notes:

• ду́маю has stress on the first syllable
• э́том has stress on э́
• ка́ждый has stress on the first syllable
• день ends in a soft consonant

It is neutral and works in both formal and informal Russian.

Nothing in the sentence is slangy, overly formal, or literary. It is just a normal everyday sentence.

So you could use it:

• in conversation
• in writing
• in class examples
• in many ordinary contexts

Yes, depending on context.

Russian present tense often covers both:

• I think about it every day
• I am thinking about it every day

In English, the first is usually more natural for a habitual action, because of every day. Russian думаю can express that habitual meaning without needing a separate continuous form.

So the most natural English translation is usually:

• I think about it every day.