L’excès d’eau passe par le trou de drainage.

Because le becomes l’ before a vowel sound. French avoids the clash of two vowel sounds together, so le excès changes to l’excès.

This is called elision. You also see it in:

• l’eau
• l’homme
• l’idée

So l’excès still means the excess; it is just the shortened form of le.

Here, excès de means an excess of or too much of something. After excès de, French normally uses de directly before the noun.

So:

• un excès d’eau = an excess of water
• un excès de sucre = too much sugar
• un excès de vitesse = speeding / excess speed

You would not normally say un excès de l’eau here, because the phrase is not talking about the water as a specific separate object; it is expressing quantity or excess.

Passer par means to go through, to pass through, or to go by way of, depending on context.

In this sentence:

• passe = passes / goes
• par = through / by

So passe par le trou de drainage means that the excess water goes through the drainage hole.

This is a very common structure:

• Le train passe par Lyon. = The train goes through Lyon.
• L’air passe par le filtre. = The air passes through the filter.

Le trou means the hole, while un trou means a hole.

French often uses the definite article when the object is already known from the situation or context. If you are talking about a plant pot, a sink, or a container that has a specific drainage hole, le trou de drainage makes sense as the drainage hole.

If the speaker were introducing it for the first time as just any hole, un trou de drainage could also be possible in another context.

De drainage specifies what kind of hole it is: a drainage hole.

French often uses de + noun where English uses a noun directly before another noun. So:

• un trou de drainage = a drainage hole
• une bouteille d’eau = a water bottle
• une salle de bain = a bathroom

In English, nouns often stack together. In French, that relationship is frequently shown with de.

Yes. Drainage is a real French noun, and it is commonly used in technical, gardening, medical, and everyday practical contexts.

For example:

• un système de drainage = a drainage system
• le drainage du sol = soil drainage

So trou de drainage is a natural phrase, especially in instructions or descriptions involving pots, containers, or water flow.

Because the subject is L’excès d’eau, which is singular.

The main noun is excès, and here it refers to the excess amount as one thing. So French uses the singular form of passer:

• il/elle passe

Even though water is something uncountable, the grammatical subject is not eau alone. It is l’excès d’eau, which is treated as singular.

A careful pronunciation would be approximately:

Lex-say do pass par luh troo duh dray-nazh

A few useful points:

• L’excès sounds like lek-sè
• d’eau sounds like do
• passe sounds like pass
• trou sounds roughly like troo
• drainage sounds like dray-nazh

Also notice the flow:

• L’excès d’eau links very smoothly because of the apostrophes and vowel sounds.

They show elision, where a vowel is dropped before another vowel sound.

• le + excès becomes l’excès
• de + eau becomes d’eau

French does this very often for smoother pronunciation. Other examples:

• je aime becomes j’aime
• le arbre becomes l’arbre
• de accord becomes d’accord

So the apostrophe is not optional here; it is the normal spelling.

Yes, that is possible, but it sounds a little different.

• L’excès d’eau focuses on the excess amount of water
• L’eau en excès focuses more on the water that is in excess

Both can work, but L’excès d’eau passe par le trou de drainage is very natural in explanatory or instructional language, especially when describing what happens to extra water in a pot or container.