Intersecția mare este periculoasă iarna.

In Romanian, the default position of most adjectives is after the noun:

• intersecția mare = the big intersection
• casă veche = old house
• mașină roșie = red car

So intersecția mare is the normal, neutral word order.

Some adjectives can appear before the noun, but that usually adds nuance (emphasis, a more literary tone, or a slightly different meaning). With mare, both:

• intersecția mare
• marea intersecție

are possible (see another question below), but intersecția mare is more neutral and common in everyday speech.

The -a at the end of intersecția is the definite article (“the”).

Romanian usually attaches the definite article to the end of the noun:

• intersecție = (an) intersection
• intersecția = the intersection

Other examples:

• casă → casa (house → the house)
• mașină → mașina (car → the car)

So intersecția mare literally is “the intersection big”, i.e. “the big intersection”.

Because the noun is singular.

Adjectives agree with the noun in number (singular/plural):

• mare = singular (both masculine and feminine)
• mari = plural (both masculine and feminine)

Here we have:

• intersecția – singular, so
• mare – singular

If it were plural (“the big intersections”), it would be:

• intersecțiile mari – the big intersections

Periculoasă is the feminine singular form of the adjective periculos (“dangerous”).

Romanian adjectives agree with the noun in gender and number:

• masculine singular: periculos
• feminine singular: periculoasă
• plural (masc. & fem.): periculoși / periculoase (more detailed, but not needed here)

The noun intersecție is feminine, so:

• intersecția → feminine singular
• periculoasă → feminine singular form to match it

So we say:

• Intersecția mare este periculoasă.
  The big intersection is dangerous.

In Romanian, every noun has a grammatical gender: masculine, feminine, or neuter.

Intersecție belongs to the feminine class. You can tell because:

• its basic form ends in -ie (a common feminine ending)

• its indefinite article is o (the feminine “a/an”):

  • o intersecție = an intersection
  • intersecția = the intersection

Because it’s feminine singular, any adjectives that describe it must take feminine singular forms when they change for gender, like periculoasă.

With seasons, Romanian very often uses the definite form of the noun without a preposition to mean “in (the) X season”:

• iarna = in (the) winter
• vara = in (the) summer
• toamna = in (the) autumn/fall
• primăvara = in (the) spring

So:

• Intersecția mare este periculoasă iarna.
  = The big intersection is dangerous in winter.

You can say:

• în iarnă – in a (particular) winter / in winter (more specific or stylistic)
• în timpul iernii – during the winter

but for a general, habitual statement (“in winter (as a rule)”), iarna on its own is the most natural.

Romanian and English use the definite article differently.

In phrases of time with seasons, Romanian typically uses the definite form:

• iarna (literally “the winter”)
• vara, toamna, primăvara

But in English, we normally say:

• “in winter”, “in summer”, “in autumn”, “in spring”
  (no the in general statements)

So the Romanian definite article -a on iarna doesn’t always correspond to an English the. You translate it by meaning:

• iarna → “in winter” (general)
• în iarna asta → “this winter” (now the “the + this” is explicit in both)

Grammatically, iarna is a noun (“winter”) in the definite singular form.

Functionally, in this sentence it acts as an adverbial of time, answering:

• When? → iarna (in winter)

Romanian often uses bare nouns (especially days, months, seasons) in exactly this way, functioning like adverbs:

• Duminica dorm mult. – On Sundays I sleep a lot.
• Toamna plouă des. – In autumn it rains often.
• Iarna este frig. – It is cold in winter.

Yes:

• Full form: Intersecția mare este periculoasă iarna.
• Common spoken form: Intersecția mare e periculoasă iarna.

e is a short, colloquial form of este, similar to “’s” in English (“is” → “’s”).

In spoken Romanian, e is very frequent; in writing, este is more neutral and slightly more formal.
You cannot drop the verb entirely here; you need either este or e.

Both are grammatically correct and both mean “The big intersection is dangerous in winter.”

• Intersecția mare – neutral, everyday order: noun + article + adjective
• Marea intersecție – adjective with article before the noun

When the adjective comes before the noun and bears the article (marea, marele, etc.), it often sounds:

• more emphatic or stylistic, or
• a bit more literary or dramatic.

So:

• Intersecția mare este periculoasă iarna. – neutral description.
• Marea intersecție este periculoasă iarna. – could feel more like “that big intersection (you know which one) is dangerous in winter”, possibly with extra emphasis.

In everyday speech, intersecția mare is more common.

You’d need to change the noun, the article, both adjectives, and the verb to the plural:

• Intersecțiile mari sunt periculoase iarna.
  
  • intersecții → intersections
  • intersecțiile → the intersections
  • mari → plural of mare
  • sunt → are (3rd person plural of a fi)
  • periculoase → feminine plural form of periculos / periculoasă

So:

• Intersecțiile mari sunt periculoase iarna.
  = The big intersections are dangerous in winter.

Sentence: Intersecția mare este periculoasă iarna.

• Subject: Intersecția mare – the big intersection
• Predicate (verb + complement): este periculoasă iarna – is dangerous in winter

Inside the predicate:

• este – the verb “to be” (3rd person singular)
• periculoasă – predicate adjective (what the subject is)
• iarna – adverbial of time (when?).

Approximate pronunciations (using English-like spelling):

• Intersecția: een-ter-SEK-tsya

  • ț = like ts in “cats”
  • ia at the end = “ya”

• periculoasă: pe-ree-koo-LOA-să

  • cu = “koo”
  • oa = like “wa” in “water” (British-like “woa”)
  • final ă is a short, neutral vowel (like the a in “sofa”)

So the whole sentence sounds roughly like:
Een-ter-SEK-tsya mare yeste pe-ree-koo-LOA-să YAR-na.