Ova situacija je problem.

Because ova has to agree with situacija in gender, number, and case.

• situacija is feminine
• it is singular
• here it is in the nominative

So the correct form of this is ova.

Compare:

• ova situacija = this situation
• ovaj problem = this problem
• ovo pitanje = this question

So English this has one form, but Croatian changes the form to match the noun.

Usually, yes. In Croatian, nouns ending in -a are very often feminine, and situacija is one of them.

So:

• situacija = feminine singular noun
• nominative singular form = situacija

That is why it combines with ova and with the singular verb je.

There are some exceptions in Croatian, but for a learner, -a often means feminine is a very useful rule.

Je means is. It is the 3rd person singular present form of the verb biti = to be.

So:

• ja sam = I am
• ti si = you are
• on/ona/ono je = he/she/it is

In this sentence, situacija is singular, so Croatian uses je.

Because after the verb biti in a sentence like this, Croatian normally uses the nominative for the predicate noun as well.

So both parts are in the nominative:

• ova situacija = nominative
• problem = nominative

That is why the sentence is:

• Ova situacija je problem.

and not:

• Ova situacija je problema.
• Ova situacija je problemom.

Those other forms belong to other cases and would not fit here.

Because Croatian does not have articles like English a/an/the.

So Croatian simply says:

• problem

and English translates that as:

• a problem
• sometimes the problem, depending on context

In this sentence, English naturally uses a problem, but Croatian does not need a separate word for that.

You can, but it usually changes the tone.

• jedan literally means one
• sometimes it can feel like a certain or add emphasis

So:

• Ova situacija je problem. = the normal, neutral sentence
• Ova situacija je jedan problem. = possible, but more marked and less natural as a basic translation

For a simple statement, problem by itself is the best choice.

The sentence Ova situacija je problem. is the neutral word order.

Croatian word order is fairly flexible, but not completely free. A key point here is that je is a clitic, so it usually comes in second position.

Neutral order:

• Ova situacija je problem.

Possible with different emphasis:

• Problem je ova situacija. = emphasizes problem

What is not normal in standard everyday usage is moving je around freely, for example:

• Ova situacija problem je. = unnatural in ordinary speech

So yes, word order can change, but je follows special placement rules.

Yes. Problem is a masculine noun.

A very common pattern in Croatian is:

• nouns ending in a consonant are often masculine

So:

• problem = masculine singular

That is why, if you put a demonstrative or adjective with it, you would say:

• ovaj problem = this problem
• velik problem = a big problem

Yes, that is grammatically possible, but it changes the feel slightly.

• Ova situacija je problem. = This situation is a problem.
• Situacija je problem. = The situation / situation is a problem, depending on context

So ova makes the reference more specific and explicit. It points to this particular situation.

Yes, but the meaning changes.

• ova situacija = this situation
• ta situacija = that situation
• ona situacija = that situation over there / that one

All three must still agree with situacija, so they use the feminine singular forms:

• ova
• ta
• ona

So in your sentence, ova is correct if the intended meaning is this situation.