Светофор на главной улице показывает зелёный цвет.

Because the preposition на here means on / in / at a location, and when it expresses location, Russian normally uses the prepositional case.

• главная улица = main street (dictionary form, nominative)
• на главной улице = on the main street

Both words change:

• главная → главной
• улица → улице

So на главной улице is the correct way to say on the main street.

Because главной is the feminine singular prepositional form of the adjective главный (main).

The noun улица is:

• feminine
• singular
• in the prepositional case

So the adjective must match it:

• главная улица — nominative
• на главной улице — prepositional

This is an example of adjective-noun agreement in Russian.

Because улица is a feminine noun, and in the prepositional singular, many nouns of this type change -а to -е.

So:

• улица → улице

This is a very common pattern:

• комната → в комнате
• школа → в школе
• улица → на улице

Показывает is the 3rd person singular present tense of показывать (to show).

The subject is светофор (traffic light), which is singular, so the verb must also be singular:

• я показываю — I show
• ты показываешь — you show
• он / она / оно показывает — he / she / it shows

Here, светофор показывает means the traffic light shows.

Because зелёный цвет is the direct object of the verb показывает, and with this verb, the direct object usually goes in the accusative case.

However, цвет is a masculine inanimate noun, and masculine inanimate nouns have the same form in the accusative as in the nominative.

So:

• nominative: зелёный цвет
• accusative: зелёный цвет

That is why it looks unchanged.

Yes, often it can, especially in everyday speech.

A native speaker might say:

• Светофор показывает зелёный.

This is natural and common, because the noun цвет is easy to understand from context.

The version with зелёный цвет is a bit more explicit and can sound slightly more formal, careful, or textbook-like.

Yes, Russian word order is more flexible than English word order.

This sentence uses a neutral, straightforward order:

• Светофор — subject
• на главной улице — location
• показывает — verb
• зелёный цвет — object

But other orders are possible, depending on emphasis. For example:

• На главной улице светофор показывает зелёный цвет.
  This puts more focus on the location.

• Зелёный цвет показывает светофор на главной улице.
  Grammatically possible, but less neutral and more marked.

So the given version is a normal, clear default sentence.

Here на means something like on or in the area of a street.

In Russian, with streets, squares, and similar public places, на is often used:

• на улице — on the street
• на площади — in/on the square
• на проспекте — on the avenue

By contrast, в is more commonly used with enclosed spaces or larger areas:

• в доме — in the house
• в городе — in the city
• в комнате — in the room

So на главной улице is the normal choice.

The correct pronunciation is with ё: зелёный.

In many Russian texts, especially informal or non-teaching materials, ё is often written as е, so you may see:

• зеленый

But it is still pronounced зелёный, not зеленый with a plain e sound.

For learners, it is helpful when ё is written, because it shows both:

• the correct vowel sound
• the stress position

The stressed syllables are:

• светофор
• на главной улице
• показывает
• зелёный цвет

So the full sentence is pronounced approximately as:

• светофор на главной улице показывает зелёный цвет

Stress is very important in Russian because it is not always predictable.

Yes, светофор is masculine.

A good clue is that it ends in a consonant in its dictionary form:

• светофор

Many Russian nouns ending in a consonant are masculine.

That is why the verb is in the masculine-compatible singular pattern:

• светофор показывает

And if you used an adjective with it, it would also be masculine:

• новый светофор — a new traffic light

Показывать is the imperfective verb, and it fits because the sentence describes a current state or ongoing situation:

• the traffic light is showing green

Russian uses the imperfective for:

• habitual actions
• ongoing actions
• general descriptions
• present tense statements

A perfective partner such as показать usually does not have a normal present-tense meaning like this. Perfective present forms usually refer to the future.

So for a simple present description, показывает is the natural choice.

Not exactly word-for-word, but in context it communicates the same idea.

Literally:

• зелёный цвет = green color

But with a traffic light, it clearly means the green signal. Russian can also express this idea in other ways, for example:

• зелёный свет — green light
• зелёный сигнал — green signal

The exact phrasing can vary, but the meaning in this sentence is clear.