Perioada contractuală începe luni.

Because perioada is the definite form of perioadă.

• perioadă = a period
• perioada = the period

Romanian usually puts the definite article at the end of the noun, not as a separate word in front like English the.

So:

• perioadă contractuală = a contractual period
• perioada contractuală = the contractual period

Because the adjective has to agree with the noun it describes.

Here, the noun is perioada, which is:

• feminine
• singular
• definite

So the adjective also appears in the feminine singular form:

• masculine singular: contractual
• feminine singular: contractuală

That is why you get perioada contractuală.

In Romanian, adjectives often come after the noun. So this word order is very normal:

• perioada contractuală = the contractual period

English usually prefers adjective + noun:

• contractual period

Romanian commonly uses:

• noun + adjective

Sometimes Romanian adjectives can come before the noun, but that usually adds emphasis, style, or a slightly different tone. In this sentence, perioada contractuală is the most natural neutral order.

Începe is the 3rd person singular present tense of a începe, which means to begin / to start.

So:

• eu încep = I begin
• tu începi = you begin
• el/ea începe = he/she begins
• perioada începe = the period begins

It is singular because the subject, perioada contractuală, is singular.

Because Romanian normally does not need a preposition before days of the week in this kind of time expression.

So:

• luni = on Monday
• marți = on Tuesday
• miercuri = on Wednesday

This is very common in Romanian. English needs on, but Romanian usually does not.

It can mean either, depending on context.

• Perioada contractuală începe luni. usually means The contractual period starts on Monday.
• In other contexts, luni can also mean on Mondays.

Romanian often relies on context to show whether it is a single Monday or a repeated Monday. In this sentence, the most natural reading is on Monday, because începe describes one starting point.

Because in Romanian, the days of the week are normally written with lowercase letters, not capitals.

So:

• luni = Monday
• marți = Tuesday
• duminică = Sunday

This is different from English, where Monday must be capitalized.

A simple approximation is:

• începe ≈ uhn-CHE-pe

A few helpful points:

• î is a Romanian vowel that does not exist in English exactly. It is a close central sound, somewhat like a tight unstressed uh, but not exactly.
• ce is pronounced like che in check, so -ce- sounds like che.
• The stress is usually on the second syllable: în-CE-pe.

Because the subject is already stated: Perioada contractuală.

Romanian does not need an extra subject pronoun when the noun is already there.

So:

• Perioada contractuală începe luni. = The contractual period starts on Monday.

Adding a pronoun like ea would usually be unnecessary here.

No, that would not be natural Romanian.

For days of the week, Romanian normally says simply:

• începe luni = starts on Monday

Using în here is not the normal pattern.

So the correct sentence is:

• Perioada contractuală începe luni.

It can mean both. In many contexts, a începe translates as either to begin or to start.

So this sentence could be understood as:

• The contractual period begins on Monday.
• The contractual period starts on Monday.

In everyday use, both are good English translations.