Το υπόστρωμα είναι εντελώς ξηρό.

Το is the definite article, meaning the.

In this sentence, το goes with υπόστρωμα and shows that the noun is:

• neuter
• singular
• here, functioning as the subject

So το υπόστρωμα = the substrate / the base layer / the underlayer, depending on context.

Υπόστρωμα literally refers to something like an underlayer, base layer, or substrate.

Its exact translation depends on context. For example, it can mean:

• substrate in technical or scientific contexts
• undercoat / base layer in construction or painting
• bedding / lining material in some practical contexts

So it is a fairly technical or context-dependent noun rather than a very everyday word.

Υπόστρωμα is neuter.

One clue is its ending: many Greek nouns ending in -μα are neuter, for example:

• το γράμμα = the letter
• το όνομα = the name
• το πράγμα = the thing
• το υπόστρωμα = the substrate

That is why both the article and the adjective are also in the neuter form:

• το υπόστρωμα
• ξηρό

Because adjectives in Greek must agree with the noun they describe in:

• gender
• number
• case

Since υπόστρωμα is neuter singular, the adjective must also be neuter singular:

• masculine: ξηρός
• feminine: ξηρή
• neuter: ξηρό

So:

• το υπόστρωμα είναι ξηρό = correct

If the noun were feminine, you would need ξηρή. If it were masculine, you would need ξηρός.

Εντελώς means completely or totally.

It is an adverb, so it modifies the adjective ξηρό:

• εντελώς ξηρό = completely dry

This placement is very natural in Greek. Putting the adverb before the adjective is the normal way to intensify it.

So the structure is:

• είναι = is
• εντελώς ξηρό = completely dry

No. Εντελώς is an adverb, and adverbs do not agree with nouns.

So it stays the same no matter what noun you use:

• το υπόστρωμα είναι εντελώς ξηρό
• η επιφάνεια είναι εντελώς ξηρή
• οι τοίχοι είναι εντελώς ξηροί

Only the adjective changes; εντελώς does not.

Είναι is the verb to be in the 3rd person singular, so here it means is.

Greek normally does use the verb είναι in a full sentence like this:

• Το υπόστρωμα είναι εντελώς ξηρό.

In some very informal notes, labels, or headline-style language, it might be omitted, but in standard complete sentences it is normally present.

So for a learner, the safe rule is:

• include είναι when you mean is

The given word order is the most natural and neutral one:

• Το υπόστρωμα είναι εντελώς ξηρό.

Greek word order is somewhat flexible, but not every rearrangement sounds equally natural.

For example:

• Το υπόστρωμα είναι εντελώς ξηρό. → neutral and standard
• Το υπόστρωμα είναι ξηρό. → also fine, just without completely
• Εντελώς ξηρό είναι το υπόστρωμα. → possible, but more emphatic or marked

So yes, Greek can move things around for emphasis, but the original order is the best default pattern for learners.

A helpful approximate pronunciation is:

ee-PO-stro-ma

More specifically:

• υ sounds like ee
• the stress is on πό
• στρ is a consonant cluster, like str in street

So:

• υπόστρωμα ≈ ee-PO-stro-ma

The full sentence can be approximated as:

• to ee-PO-stro-ma EE-ne en-de-LOS ksi-RO

That is only an approximation for English speakers, but it is a useful starting point.

Greek uses accent marks to show which syllable is stressed.

In ξηρό, the accent shows that the stress falls on the final syllable:

• ξη-ΡΟ

Likewise:

• υΠΟστρωμα
• εντεΛΩΣ
• ξηΡΟ

Learning to notice accent marks is very important in Greek, because stress is part of correct pronunciation.

Sometimes yes, but the choice depends on context.

Both can relate to dry, but they are not always identical:

• ξηρός / ξηρό often means dry in a general or material sense
• στεγνός / στεγνό often emphasizes not wet, especially after water or moisture

In a technical sentence about a material, surface, or substrate, ξηρό is very natural.
So Το υπόστρωμα είναι εντελώς ξηρό sounds perfectly appropriate.

A learner should understand that Greek may choose different words for dry depending on the situation.

It is in the nominative, because it is the subject of the sentence:

• Το υπόστρωμα = the thing being talked about
• είναι = is
• εντελώς ξηρό = completely dry

One detail that can confuse learners: for many neuter nouns, the nominative and accusative singular look the same. So το υπόστρωμα could look identical in both cases, but here its role in the sentence tells you it is nominative.