Sa ilalim ng tulay ang ilog, at may mga bisikleta sa gilid ng bangketa.

Because Filipino often puts the predicate first and the topic after it.

In Sa ilalim ng tulay ang ilog:

• Sa ilalim ng tulay = the location/predicate
• ang ilog = the topic, the river

So the structure is closer to:

• Under the bridge is the river
• rather than the more English-like The river is under the bridge

This word order is very normal in Filipino.

There is no missing word. Filipino often does not need a verb like is/are in this kind of sentence.

So:

• Sa ilalim ng tulay ang ilog literally works without a separate word for is
• It naturally means The river is under the bridge

This is a common pattern in Filipino equational or descriptive sentences.

It means under the bridge.

Breakdown:

• sa = in, at, on, to, under, etc. depending on context; a general location marker
• ilalim = underside, lower part, beneath
• ng tulay = of the bridge

So sa ilalim ng tulay literally means at the underside of the bridge, which is naturally translated as under the bridge.

Here ng links one noun to another, often like of in English.

Examples from the sentence:

• ilalim ng tulay = underside of the bridge
• gilid ng bangketa = side/edge of the sidewalk

So in these phrases, ng is not marking a direct object. It is acting more like a linker meaning of.

Ang marks the topic or focus of the clause.

In Sa ilalim ng tulay ang ilog, the thing being identified or talked about is ang ilog = the river.

It does not work exactly like English the, although it is often translated that way. Its job is grammatical: it marks the noun as the topic of the sentence.

May means there is/there are or has/have, depending on context.

Here:

• may mga bisikleta = there are bicycles

So the second clause:

• may mga bisikleta sa gilid ng bangketa means
• there are bicycles at the side of the sidewalk

This is a very common way to express existence in Filipino.

Mga marks plurality.

So:

• bisikleta = bicycle / bicycles, depending on context
• mga bisikleta = bicycles

Unlike in English, the noun itself usually does not change form. The plural idea is shown by mga.

So:

• may bisikleta = there is a bicycle / there are bicycles, depending on context
• may mga bisikleta = there are bicycles, clearly plural

Because many Filipino nouns do not change their form for singular vs. plural. Instead, plurality is often shown by mga.

So:

• bisikleta can refer to bicycle or bicycles
• mga bisikleta makes it clearly plural

This is very different from English, where we usually add -s.

It means at the side of the sidewalk or by the edge of the sidewalk.

Breakdown:

• sa = at/in/on/by
• gilid = side, edge
• ng bangketa = of the sidewalk

So the whole phrase is a location phrase telling you where the bicycles are.

At means and.

It connects the two parts of the sentence:

• Sa ilalim ng tulay ang ilog
• at may mga bisikleta sa gilid ng bangketa

So the whole sentence joins two ideas:

• the river is under the bridge
• and there are bicycles by the sidewalk

Yes. Nasa ilalim ng tulay ang ilog is also natural.

Nasa is basically na + sa, and in many everyday sentences it is used for location:

• Nasa ilalim ng tulay ang ilog = The river is under the bridge

Both versions are understandable, but nasa often sounds especially natural in conversational Filipino when stating where something is.

Yes, bangketa means sidewalk or pavement.

So:

• sa gilid ng bangketa = by the side of the sidewalk

It is a very common everyday word in Filipino.

Because the two clauses are doing different things.

1. Sa ilalim ng tulay ang ilog

   • identifies the location of a specific topic: the river

2. may mga bisikleta sa gilid ng bangketa

   • states that something exists in a place: there are bicycles by the sidewalk

So the first is a topic-predicate type sentence, while the second uses may for existence.

Yes, that is also possible.

• Sa ilalim ng tulay ang ilog
• Ang ilog ay sa ilalim ng tulay

Both can mean The river is under the bridge.

The version in your sentence is more predicate-first, which is very common. The ay version is more explicitly structured and can sound more formal or more carefully arranged.

Yes, it is grammatical and understandable.

A learner should mainly notice these natural features:

• location phrase first: Sa ilalim ng tulay
• no separate word for is
• existence expressed with may
• plurality marked by mga
• noun-linking with ng

A very conversational version might sometimes use nasa in the first clause, but the sentence as given is still good Filipino.