Regula este simplă.

Because regula means the rule, not just rule.

In Romanian, the definite article is usually attached to the end of the noun instead of appearing as a separate word like English the.

• regulă = a rule / rule
• regula = the rule

So Regula este simplă literally follows the Romanian pattern rule-the is simple.

Romanian usually uses a postposed definite article, meaning the definite article is added to the end of the noun.

For feminine singular nouns like regulă, the definite form is often made by adding -a:

• o regulă = a rule
• regula = the rule

This is one of the big differences from English.

Because the adjective has to agree with the noun in gender and number.

Regula is a feminine singular noun, so the adjective must also be feminine singular:

• masculine singular: simplu
• feminine singular: simplă

So:

• un lucru simplu = a simple thing
• o regulă simplă = a simple rule
• Regula este simplă = The rule is simple

You usually learn the gender along with the noun. A very common clue is the indefinite article:

• o regulă = feminine
• un
  • noun = usually masculine or neuter in the singular

Also, many Romanian nouns ending in -ă are feminine, though you should not rely on endings alone every time.

Because regulă is feminine, related words also show feminine forms:

• o regulă
• regula
• simplă

In standard Romanian, the verb a fi (to be) is normally expressed in sentences like this.

So:

• Regula este simplă = The rule is simple

Romanian generally does not omit is in normal neutral statements the way Russian or Arabic might in the present tense.

That said, in casual speech, people may sometimes leave out este in certain styles or headlines, but for a normal learner sentence, este is the standard and correct form.

Yes. E is a very common short form of este in everyday Romanian.

Both are correct:

• Regula este simplă = more full/formal/neutral
• Regula e simplă = very common in speech, slightly more conversational

As a learner, you should recognize both.

The word order is:

Subject + verb + adjective/complement

• Regula = subject
• este = verb
• simplă = predicate adjective

So Romanian is behaving very similarly to English here:

• The rule / is / simple

This is the most natural neutral order for this sentence.

You could, but it sounds marked or stylistically unusual. It might be used for emphasis, contrast, or a literary effect.

The normal order is:

• Regula este simplă.

A fronted adjective like Simplă este regula is not what a learner should use as the default.

A helpful approximation is:

REH-goo-la

More carefully in Romanian:

• re as in a short reh
• gu with a hard g, like in good
• la as in lah

The stress is on the first syllable: RE-gu-la.

A rough pronunciation is:

SEEM-pluh

The final ă is a very common Romanian vowel, called schwa. It sounds like the unstressed a in English sofa.

So simplă is not simpla with a full ah sound. The ending is softer:

• simplă ≈ SEEM-pluh

This ă sound is extremely important in Romanian, so it is worth getting used to early.

In this sentence, simplă most naturally means simple.

Depending on context, simple and easy can overlap somewhat in English, but Romanian usually keeps them separate:

• simplu / simplă = simple
• ușor / ușoară = easy / light

So Regula este simplă means the rule is straightforward or uncomplicated, not necessarily easy in every sense.

The indefinite version would be:

• O regulă este simplă. = A rule is simple.

More naturally, Romanian might also say:

• O regulă simplă = a simple rule

Compare:

• regulă = rule
• o regulă = a rule
• regula = the rule

The plural forms are:

• regulă = rule
• reguli = rules
• regula = the rule
• regulile = the rules

And the adjective changes too:

• simplă = feminine singular
• simple = feminine plural

So:

• Regulile sunt simple. = The rules are simple.

Notice that the verb also changes:

• este = is
• sunt = are

Yes. Romanian uses regulă in several common expressions. One very useful one is:

• în regulă = OK / all right / in order

For example:

• Totul este în regulă. = Everything is OK.

But in Regula este simplă, regula is just the ordinary noun meaning the rule.