Il pendio è molto ripido.

Because pendio is a masculine singular noun, and it begins with p, not with a sound that requires lo.

In Italian:

• il is used with most masculine singular nouns:

  • il libro
  • il ragazzo
  • il pendio

• lo is used before certain masculine singular nouns beginning with:

  • s + consonant: lo studente
  • z: lo zaino
  • ps: lo psicologo
  • gn: lo gnomo
  • x, y, etc.

So pendio takes il, not lo.

Pendio is a noun. It is a masculine singular noun, which is why it takes il and why the adjective appears as ripido.

A useful pattern to notice:

• il pendio ripido = the steep slope
• i pendii ripidi = the steep slopes

The accent shows that è means is.

This is important because:

• è = is (from the verb essere, to be)
• e = and

So:

• Il pendio è molto ripido. = The slope is very steep.
• Il pendio e la strada... = The slope and the road...

In writing, the accent is necessary to distinguish the two.

È is the third-person singular present of essere (to be).

Here is the present tense of essere:

• io sono = I am
• tu sei = you are
• lui/lei è = he/she is
• noi siamo = we are
• voi siete = you are
• loro sono = they are

In Il pendio è molto ripido, the subject is il pendio (the slope), which is singular, so Italian uses è.

Because molto here means very, not much.

In Italian, molto can work in different ways:

• as an adverb meaning very:
  molto ripido = very steep
• as an adjective meaning much/a lot of, where it changes form:
  • molto tempo = much time
  • molta acqua = a lot of water
  • molti libri = many books
  • molte case = many houses

In your sentence, molto modifies the adjective ripido, so it stays molto.

Because ripido agrees with pendio, which is masculine singular.

Italian adjectives usually agree in gender and number with the noun they describe.

Here are the main forms:

• ripido = masculine singular
• ripida = feminine singular
• ripidi = masculine plural
• ripide = feminine plural

Examples:

• Il pendio è ripido.
• La salita è ripida.
• I pendii sono ripidi.
• Le strade sono ripide.

Yes. Ripidissimo means very steep or extremely steep.

Italian often uses the suffix -issimo to strengthen an adjective:

• ripido = steep
• ripidissimo = very / extremely steep

So both are correct, but the nuance is slightly different:

• molto ripido = very steep
• ripidissimo = extremely steep / super steep

Both are natural, though molto ripido is usually more neutral.

The normal word order is:

Il pendio è molto ripido.

That is:

subject + verb + adverb + adjective

This is the most natural neutral order.

Italian can sometimes move things around for emphasis, but that is less basic. For a learner, the standard order is best.

It is pronounced roughly PEN-dyo.

A few pronunciation notes:

• pen- sounds like pen
• -dio here sounds like dyo, with io forming a glide
• the stress is on the first syllable: PEN-dio

So not pen-DEE-oh, but closer to PEN-dyo.

It is pronounced roughly REE-pee-do.

Notes:

• ri = ree
• pi = pee
• do = do
• the stress is on the first syllable: RI-pi-do

So it sounds like REE-pee-do, not ri-PI-do.

Normally, no. In standard Italian, you usually need the definite article with a common noun used as the subject:

• Il pendio è molto ripido.

Leaving out il would sound incomplete or unnatural in this context.

Italian uses articles more often than English does, so even when English says slope, Italian often wants il pendio or un pendio, depending on the meaning.

The plural is:

I pendii sono molto ripidi.

Changes:

• il → i
• pendio → pendii
• è → sono
• ripido → ripidi

So:

• Il pendio è molto ripido. = The slope is very steep.
• I pendii sono molto ripidi. = The slopes are very steep.

Yes. Those are natural alternatives with slightly different tones:

• molto ripido = very steep
• assai ripido = quite/very steep, a bit more literary or formal
• davvero ripido = really steep
• estremamente ripido = extremely steep

So molto ripido is the safest everyday choice, but the others are also possible depending on style and emphasis.