הצינור מתחת לכיור ישן, ולכן כדאי להחליף אותו.

A common transliteration is:

ha-tsinor mitakhat la-kiyor yashan, ve-lakhen כדאי le-hakhlif oto

A more fully Hebrew-based transliteration would be:

ha-tzinor mitáchat la-kiyor yashan, ve-lakhen kedai lehakhlif oto

Natural pronunciation:

• הצינור = ha-tsi-NOR
• מתחת = mi-TA-khat / mi-TA-chat
• לכיור = la-ki-YOR
• ישן = ya-SHAN
• ולכן = ve-la-KHEN
• כדאי = ke-DAI
• להחליף = le-ha-khLIF
• אותו = o-TO

In present-tense Hebrew, the verb to be is usually omitted.

So:

• הצינור ישן literally looks like the pipe old
• but it means the pipe is old

This is completely normal Hebrew. English needs is, but Hebrew usually does not in the present tense.

Because הצינור is a masculine singular noun, the adjective has to match it.

So:

• צינור = masculine singular
• ישן = masculine singular form of old

If the noun were feminine singular, you would use ישנה.

For example:

• הדלת ישנה = the door is old

Good question: the same spelling, ישן, can represent two different words:

• יָשָׁן = old
• יָשֵׁן = sleeping

In this sentence, it clearly means old, because a pipe can be old, but it cannot be sleeping.

So:

• הצינור ישן = the pipe is old
• not the pipe is sleeping

מתחת ל־ is a fixed expression meaning under / underneath / below.

So:

• מתחת by itself is not usually enough before a noun
• you normally say מתחת ל... = under ...

Therefore:

• מתחת לכיור = under the sink

You can think of it as learning the chunk מתחת ל־ as one unit.

The ל here is not really the English preposition to in meaning. In this expression, מתחת ל־, the ל־ is just part of how Hebrew connects under to the following noun.

So although ל־ often means to, in this sentence you should understand the whole phrase together:

• מתחת ל־ = under
• מתחת לכיור = under the sink

This is one of those places where translating word-by-word is less helpful than learning the full pattern.

Because the noun is definite: the sink.

Hebrew often combines a preposition with the definite article ה־. Here:

• ל
  • הכיור
• becomes לכיור in normal spelling without vowel marks

So:

• כיור = a sink
• הכיור = the sink
• לכיור here = to/for the sink in form, but as part of מתחת ל־, it means under the sink

This kind of contraction is very common in Hebrew.

ולכן means and therefore, and so, or simply so.

It is made of:

• ו־ = and
• לכן = therefore / so

So the sentence structure is:

• The pipe under the sink is old
• so / therefore it is advisable to replace it

כדאי is a very common word meaning something like:

• it is worth it
• it is advisable
• it is a good idea

In this sentence:

• כדאי להחליף אותו = it’s advisable to replace it / it would be a good idea to replace it

A very common Hebrew pattern is:

• כדאי + infinitive

For example:

• כדאי לבדוק = it’s worth checking
• כדאי לחכות = it’s better to wait
• כדאי להחליף אותו = it’s a good idea to replace it

Because after כדאי, Hebrew normally uses the infinitive.

So:

• להחליף = to replace

This is exactly the form you expect after כדאי:

• כדאי להחליף = it is advisable to replace

The base verb is:

• החליף = he replaced
• להחליף = to replace

אותו means him or it as a direct object, depending on context.

Here it refers back to הצינור = the pipe, so in English it becomes it:

• להחליף אותו = to replace it

Because צינור is masculine singular, the pronoun is אותו.

Compare:

• masculine singular: אותו = him/it
• feminine singular: אותה = her/it

So if the noun were feminine, you would use אותה instead.

Because אותו already functions as the object form by itself.

With a full definite noun, Hebrew often uses את:

• להחליף את הצינור = to replace the pipe

But with an object pronoun, you say:

• להחליף אותו = to replace it

So you do not say:

• להחליף את אותו for this meaning

In modern Hebrew, אותו itself is the normal direct-object pronoun here.