Learn Hardcore Greek: Σκέφτομαι ότι το πρόβλημα είναι σημαντικό, αλλά δεν είναι δύσκολο. - I think that the problem is important, but it is not difficult.

No. Σκέφτομαι is one of many Greek “middle-only” (deponent) verbs: it looks middle/passive but has an active meaning. Think of it as the normal way to say “I think/I'm thinking.”

Present (indicative): σκέφτομαι, σκέφτεσαι, σκέφτεται, σκεφτόμαστε, σκέφτεστε, σκέφτονται
Aorist (simple past): σκέφτηκα, σκέφτηκες, σκέφτηκε, σκεφτήκαμε, σκεφτήκατε, σκέφτηκαν You don’t add a reflexive pronoun; the -μαι is built into the verb.

Is σκέφτομαι the best verb for “I think” (meaning “I believe”)?

It can be, but more common for opinions is νομίζω (ότι/πως) “I think/I believe,” or θεωρώ “I consider.”

Σκέφτομαι ότι… often suggests “I’m thinking/reflecting that…,” focusing on the mental process.
Νομίζω ότι… simply states an opinion.
Θεωρώ ότι… / Θεωρώ X Y is a bit more formal (“I consider X (to be) Y”).

Do I have to use ότι here? Could I use πως or drop it?

After verbs of thinking/saying, use ότι or πως to introduce a content clause. In this sentence, ότι and πως are interchangeable.
Don’t confuse πως (that) with πώς (how) — the question word has an accent.
In careful speech/writing, keep ότι/πως. In casual speech, some speakers drop it with certain verbs (especially νομίζω), but with σκέφτομαι it’s safer and more natural to keep it.

What’s the difference between ότι and ό,τι?

ότι (no comma) = “that” (a conjunction introducing a clause).
ό,τι (with a comma) = “whatever/anything that.”
They sound the same; the comma changes the meaning. Here you need ότι.

Why is it το πρόβλημα and not just πρόβλημα or ένα πρόβλημα?

Greek uses the article more than English.

το πρόβλημα = “the problem” (specific/known from context).
ένα πρόβλημα = “a problem” (non-specific).
A bare noun (πρόβλημα) is possible in certain generic or predicate uses, but here you’re talking about a specific problem, so το is right.

Why are σημαντικό and δύσκολο in the neuter form?

Adjectives agree with the noun in gender, number, and case. πρόβλημα is neuter singular, so the predicate adjectives are neuter singular too: σημαντικό, δύσκολο.
Typical patterns:

Masculine: σημαντικός, δύσκολος
Feminine: σημαντική, δύσκολη
Neuter: σημαντικό, δύσκολο

Why is the negation δεν and not μην?

δεν is used with the indicative mood (ordinary statements/questions), as in δεν είναι.
μη(ν) is used with the subjunctive (after να), with negative commands, and in a few set uses.
Examples:
Indicative: Δεν είναι δύσκολο.
Subjunctive: Να μην είναι δύσκολο.
Command: Μην το κάνεις.

Where does δεν go? Could I say είναι δεν δύσκολο?

No. The negative particle comes directly before the verb: δεν είναι.
In casual speech the final -ν may drop before many consonants, so you might hear δε before a consonant-initial word, but in writing you’ll usually see δεν.

Why is there a comma before αλλά?

Greek typically uses a comma before αλλά when it joins two full clauses, much like English. In very short contrasts it can be omitted, but using it is clear and safe.
Synonyms: μα (more informal), όμως (“however,” adverbial).

Do I have to repeat είναι in the second clause? Could I say “..., αλλά δεν δύσκολο”?

You need the verb: ..., αλλά δεν είναι δύσκολο.
If you want to avoid repeating the verb, you can make it elliptical with όχι:

..., αλλά όχι δύσκολο.
That’s fine in shorter, more stylistic sentences.

Can the word order change?

Greek allows some flexibility for emphasis, but beginners should keep the original order. Acceptable variants:

Σκέφτομαι πως το πρόβλημα είναι σημαντικό, αλλά δεν είναι δύσκολο. (using πως)
Σκέφτομαι ότι είναι σημαντικό το πρόβλημα, αλλά δεν είναι δύσκολο. (focus on “the problem”)
A heavy topicalization like Το πρόβλημα σκέφτομαι ότι είναι σημαντικό is possible but feels marked/literary.

What case do we use around είναι? Why no article with the adjectives?

With είναι, both subject and predicate are in the nominative: το πρόβλημα (nominative subject) — σημαντικό/δύσκολο (predicate adjectives). Predicate adjectives take no article.
Compare:

Attributive: το σημαντικό πρόβλημα (the important problem)
Predicative: το πρόβλημα είναι σημαντικό (the problem is important)

How do I pronounce the tricky parts?

Σκέφτομαι: SKÉF-to-me (the φτ is [ft])
ότι: Ó-ti
πρόβλημα: PRÓ-vli-ma (the βλ cluster is like “vli”)
είναι: Í-ne (ει = [i])
σημαντικό: si-man-ti-KÓ
αλλά: a-LLÁ (double λ but a normal [l] sound)
δύσκολο: DÍ-sko-lo
Stress the syllables with the written accent.

Is είναι singular or plural here?

είναι serves as both 3rd singular and 3rd plural in the present. The subject tells you which: το πρόβλημα is singular, so here it means “is.” With a plural subject (e.g., τα προβλήματα) it would mean “are.”

Could I replace ότι with που?

After many verbs of saying/perceiving in colloquial Greek, που can introduce a content clause, but after σκέφτομαι it’s much less natural. Prefer ότι or πως here: Σκέφτομαι ότι/πως…

Is there a more compact, stylistic alternative?

Yes. You can tighten the contrast by avoiding the second είναι or by using a different verb:

Σκέφτομαι ότι το πρόβλημα είναι σημαντικό, αλλά όχι δύσκολο.
Θεωρώ το πρόβλημα σημαντικό, αλλά όχι δύσκολο. (object + predicate adjective)
The second is slightly more formal and succinct.