Drenaż jest konieczny.

Because konieczny has to agree with drenaż in gender and number.

• drenaż is a singular masculine noun
• so the adjective also takes the singular masculine form: konieczny

Compare:

• zabieg jest konieczny — the procedure is necessary
• operacja jest konieczna — the operation is necessary
• badanie jest konieczne — the examination/test is necessary

So in your sentence, konieczny matches drenaż.

Drenaż is masculine.

A learner can often tell this from the dictionary form: many Polish masculine nouns end in a consonant, and drenaż ends in -ż.

That matters because other words in the sentence must match it:

• ten drenaż
• drenaż jest konieczny

In normal present-tense Polish, jest is usually used in sentences like this.

So:

• Drenaż jest konieczny. = natural, standard
• Drenaż konieczny. = possible only in very special contexts, such as notes, headlines, medical shorthand, or compressed style

For a learner, the safe rule is:

• in a full present-tense sentence meaning X is Y, use jest

The structure is:

• Drenaż — subject
• jest — 3rd person singular of być (to be)
• konieczny — predicate adjective

So this is the equivalent of:

• Drainage is necessary or
• The drainage is necessary depending on context

Polish does not use articles like a or the, so drenaż can mean either drainage or the drainage, depending on the situation.

It often means drainage, but in real usage it can be broader or more specific depending on context.

Common possibilities include:

• medical drainage / a drainage procedure
• draining fluid from the body
• technical or construction drainage

So if this sentence appears in a medical context, Drenaż jest konieczny may mean:

• Drainage is necessary
• A drain is necessary
• The drainage procedure is necessary

The exact English wording depends on the context already given.

A rough pronunciation guide is:

• drenaż ≈ DREH-nash
• jest ≈ yest
• konieczny ≈ ko-NYECH-nih

A few helpful details:

• dr is pronounced clearly, like in English dr-
• e is like e in met
• a is like a in father
• ż sounds like the s in measure
• j in Polish sounds like English y
• cz sounds like ch in chop
• ny includes a softened n sound, roughly like ny in canyon, though not exactly the same

ż is pronounced like the sound /ʐ/ in Polish, which is close to the s in English measure or vision, though usually a bit stronger and more retroflex.

So drenaż ends something like:

• -azh / -azh-like not
• -ag
• -az
• -aj

For many English speakers, DREH-nazh is a useful approximation.

Yes, you could say Konieczny jest drenaż.

Both are grammatical, but the emphasis changes a bit:

• Drenaż jest konieczny. — neutral, straightforward
• Konieczny jest drenaż. — puts more focus on necessary
• Drenaż konieczny jest. — grammatical in some literary or marked styles, but not normal everyday order

The most neutral version for learners is:

• Drenaż jest konieczny.

This is a very good question, because Polish sometimes uses the instrumental after forms of to be.

With adjectives in sentences like this, Polish normally uses the nominative:

• On jest zmęczony. — He is tired.
• To jest ważne. — This is important.
• Drenaż jest konieczny. — Drainage is necessary.

The instrumental is more relevant with many nouns after być, especially in expressions of profession, role, or identity:

• On jest lekarzem. — He is a doctor.
• Ona jest nauczycielką. — She is a teacher.

Since konieczny is an adjective, nominative is the expected form here.

Yes, often you can, but the nuance may change.

• konieczny = necessary, essential, required
• potrzebny = needed, necessary, sometimes a little less strong depending on context

So:

• Drenaż jest konieczny. — Drainage is necessary / essential.
• Drenaż jest potrzebny. — Drainage is needed.

In many practical situations they overlap, but konieczny often sounds more like there is no real alternative.

Not in this sentence, if you want to keep the same meaning.

Polish often drops subject pronouns because the verb ending shows the person:

• Jestem gotowy. — I am ready.
  No need for ja.

But here drenaż is not a pronoun; it is the actual thing being talked about. If you remove it, you get:

• Jest konieczny. — It is necessary / He is necessary

That only works if the subject is already understood from context. So on its own, you normally keep drenaż.

Yes. Jest is the 3rd person singular present tense form of być (to be).

That means it matches the singular subject drenaż.

Compare:

• drenaż jest konieczny — drainage is necessary
• zabiegi są konieczne — procedures are necessary

So if the subject changes to plural, the verb changes too:

• Drenaże są konieczne.

Usually it depends on context.

Because Polish has no articles, Drenaż jest konieczny can mean:

• Drainage is necessary — a general or abstract statement
• The drainage is necessary — a specific situation
• A drainage procedure is necessary — in a medical context

So the sentence itself is grammatically complete, but context tells you how specific it is.