Por la noche, casi no oigo el ruido de la calle.

In Spanish, por la mañana, por la tarde, and por la noche are the normal ways to say in the morning / in the afternoon-evening / at night.

So por la noche is the standard expression here.

A few comparisons:

• por la noche = at night / in the evening
• de noche = at night, when it is dark; often a bit more general
• en la noche = much less common in Spain in this kind of sentence

For a learner of Spanish from Spain, por la noche is the most natural choice.

This is a very common question.

• por la noche can refer to at night in a general sense, or to during the night/evening in a particular context.
• por las noches usually means at night(s), regularly, night after night.

So:

• Por la noche, casi no oigo el ruido de la calle. = At night, I hardly hear the noise from the street.
• Por las noches, casi no oigo el ruido de la calle. = At night / at nights / in the evenings, I hardly hear the street noise.

The plural version sounds a bit more clearly habitual.

Casi no literally means almost not, but in natural English it usually means:

• hardly
• barely
• almost never (depending on context)

Here, casi no oigo means I hardly hear or I can barely hear.

So the idea is not a complete negation, but something very close to it.

In standard Spanish, no goes directly before the conjugated verb.

• oigo = I hear
• no oigo = I do not hear
• casi no oigo = I hardly hear

That is the normal position for negation in Spanish.

English uses do not hear, but Spanish does not need an extra helping verb like do.

Because oír and escuchar are related, but not the same.

• oír = to hear
  This is about sound reaching your ears.
• escuchar = to listen
  This suggests attention or active listening.

In this sentence, the speaker is talking about whether the street noise is audible, so oigo is the best choice.

• Casi no oigo el ruido de la calle = I can hardly hear the street noise.
• Casi no escucho el ruido de la calle would sound more like I am hardly listening to it, which is less natural here.

Because oír is an irregular verb.

Its present tense forms are:

• yo oigo
• tú oyes
• él/ella/usted oye
• nosotros/nosotras oímos
• vosotros/vosotras oís
• ellos/ellas/ustedes oyen

So the yo form is oigo.

This is just something to learn as part of the conjugation of oír.

This happens mainly for pronunciation reasons.

Spanish avoids forms that would sound awkward or unclear, so oír develops y in several present-tense forms:

• oyes
• oye
• oyen

But not in:

• oímos
• oís

So the pattern is irregular, and it is best memorized as a whole.

Here de means something like from or coming from.

So:

• el ruido de la calle = the noise from the street / the street noise

This is a very common Spanish structure:

• el ruido del motor = the noise of/from the engine
• la luz del sol = the light of/from the sun
• el olor de la comida = the smell of the food

So de often shows the source or origin.

Because Spanish usually uses articles more than English does.

English often stacks nouns together:

• street noise
• car door
• office chair

Spanish normally does not do that in the same way. It often uses:

• article + noun + de + article + noun

So:

• el ruido de la calle = the noise of/from the street

That is the natural Spanish structure.

Spanish often uses definite articles where English might not.

• el ruido = the noise
• la calle = the street

Even when English says something more general like street noise, Spanish usually keeps the articles:

• el ruido de la calle

Also, ruido is masculine, so el and calle is feminine, so la.

It is not absolutely necessary, but it is perfectly natural.

The phrase por la noche is a time expression at the beginning of the sentence, and the comma helps separate it from the main statement.

So both are correct:

• Por la noche, casi no oigo el ruido de la calle.
• Por la noche casi no oigo el ruido de la calle.

The version with the comma feels slightly clearer and more written.

It is pronounced roughly OY-go.

A few points:

• oi is a diphthong, similar to the sound in English boy
• g before o is a hard g
• it has two syllables: oi-go

So a simple guide is OY-go.

In most of Spain, calle is pronounced roughly KA-ye.

Helpful details:

• ca- sounds like ka
• ll is usually pronounced like y in much of modern Spain
• the word has two syllables: ca-lle

So for most learners, KA-ye is a good target pronunciation.

Yes. Spanish word order is flexible.

The original sentence is very natural:

• Por la noche, casi no oigo el ruido de la calle.

But you could also say:

• Casi no oigo el ruido de la calle por la noche.

The meaning is basically the same, but the focus changes slightly:

• putting por la noche first gives the time setting first
• putting it at the end sounds a bit more neutral in flow

Both are correct.

The present tense in Spanish is often used for:

• things happening now
• general facts
• habits or repeated situations

Here, casi no oigo probably describes a usual situation: at night, the speaker hardly hears the street noise.

So the present tense is completely natural.