Mi multe pensas pri tiu problemo.

Pensas is the present-tense verb form, meaning think or am thinking.

In Esperanto, verbs change by ending:

• -i = infinitive, like to think
• -as = present tense
• -is = past tense
• -os = future tense

So:

• pensi = to think
• mi pensas = I think / I am thinking

Because the sentence has a subject, mi, and gives a complete statement, it needs the finite verb pensas, not the infinitive pensi.

Here multe means a lot or much, and it works as an adverb.

It modifies the verb pensas, not a noun. In other words, it tells us how much the person thinks about the problem.

Compare:

• multa problemo = a big amount of problem / much problem (adjective-like use, modifying a noun)
• mi multe pensas = I think a lot (adverbial use, modifying a verb)

The ending -e often marks adverbs in Esperanto, so multe is the natural form here.

Yes. That is also correct.

Esperanto word order is fairly flexible, as long as the sentence remains clear. Both of these are natural:

• Mi multe pensas pri tiu problemo.
• Mi pensas multe pri tiu problemo.

The first version puts slightly more emphasis on multe earlier in the sentence. The second may feel a bit closer to English word order. Both are understandable and grammatical.

Pri means about. The phrase pensi pri means to think about.

So:

• Mi pensas pri tio = I think about that
• Ŝi pensas pri la estonteco = She thinks about the future

This is just the normal preposition used with this meaning in Esperanto.

Be careful not to assume Esperanto always copies English exactly, but in this case pensi pri matches English think about very closely.

Because the phrase is the object of the preposition pri, not a direct object.

In Esperanto, the -n ending usually marks a direct object:

• Mi vidas la problemon. = I see the problem.

But after a preposition like pri, you normally do not use -n:

• Mi pensas pri la problemo.
• Mi pensas pri tiu problemo.

So tiu problemo is correct here.

Tiu usually means that or that one.

In this sentence, tiu problemo means that problem. It points to a specific problem already known in the conversation or context.

Some related words:

• tiu = that one / that
• ĉi tiu = this
• tiuj = those
• ĉi tiuj = these

So if you wanted this problem, you would say:

• Mi multe pensas pri ĉi tiu problemo.

Actually, it does match when needed.

In this sentence, both are singular and not accusative:

• tiu problemo

If the noun were plural, you would change both:

• tiuj problemoj = those problems

If accusative were needed, both would take -n:

• Mi vidas tiun problemon. = I see that problem.

So tiu behaves like other correlatives and agrees in number and accusative marking when necessary.

It is an international root rather than something borrowed directly from English alone. Esperanto often uses roots that are recognizable across many European languages.

Problemo is easy to recognize if you know words like:

• English: problem
• French: problème
• Spanish: problema
• Italian: problema

In Esperanto, nouns normally end in -o, so:

• problem- = root
• problemo = problem

Yes, very often it can.

Literally, Mi multe pensas pri tiu problemo means I think a lot about that problem, but in natural usage it can suggest:

• mental focus
• concern
• worry
• preoccupation

The exact nuance depends on context. If the topic is serious, the sentence may sound like That problem is on my mind a lot.

If someone wanted to express even stronger worry, they might choose a more specific verb, but multe pensi pri already often suggests ongoing concern.

A simple pronunciation guide for English speakers:

• Mi ≈ mee
• multe ≈ MOOL-teh
• pensas ≈ PEN-sahs
• pri ≈ pree
• tiu ≈ roughly TEE-oo (two clear vowels)
• problemo ≈ pro-BLEH-mo

A few helpful pronunciation points:

• Every vowel is pronounced clearly.
• Stress is always on the second-to-last syllable.
• u sounds like oo in food.
• e sounds like e in bet.
• i sounds like ee in see.

So the whole sentence is approximately: mee MOOL-teh PEN-sahs pree TEE-oo pro-BLEH-mo

Usually, no. Esperanto normally states the subject pronoun unless the meaning is obvious in a special context.

So the normal full sentence is:

• Mi multe pensas pri tiu problemo.

Unlike Spanish or Italian, Esperanto verbs do not change enough to show the subject clearly by themselves:

• pensas could mean I think, you think, he thinks, they think, depending on context

Because of that, the pronoun mi is usually necessary.

The sentence breaks down like this:

• Mi = subject, I
• multe = adverb, a lot
• pensas = verb, think
• pri tiu problemo = prepositional phrase, about that problem

So the structure is:

subject + adverb + verb + prepositional phrase

That gives: Mi multe pensas pri tiu problemo.

A very literal gloss would be: I much think about that problem.

But in natural English, that becomes: I think a lot about that problem.