Dưới bóng râm rất mát mẻ.

Dưới means under / below, and bóng râm means shade or shadowed area. Together, dưới bóng râm means in the shade or more literally under the shade.

A natural English translation is usually in the shade, even though Vietnamese uses dưới.

Vietnamese often does not use a separate word for to be when an adjective describes something.

So:

• rất mát mẻ = very cool / very pleasant
• The idea of is is understood automatically

That is why Dưới bóng râm rất mát mẻ can mean It is very cool in the shade without saying a separate word for is.

Because là is usually used to link nouns, not adjectives.

For example:

• Anh ấy là giáo viên. = He is a teacher.
• Trời nóng. = The weather is hot.

In your sentence, mát mẻ is an adjective, so là is normally not used.
Dưới bóng râm là rất mát mẻ would sound unnatural.

In Vietnamese, rất means very, and it normally goes before the adjective it modifies.

So:

• rất đẹp = very beautiful
• rất nóng = very hot
• rất mát mẻ = very cool / very pleasant

This is the normal Vietnamese word order.

Mát mẻ means cool, fresh, pleasantly cool. It usually suggests a comfortable feeling, not coldness.

Mát also means cool, but mát mẻ often sounds a little fuller or more descriptive, especially for weather, air, or a place.

So:

• mát = cool
• mát mẻ = pleasantly cool, fresh, nice and cool

In many situations, both are possible, but mát mẻ sounds slightly more expressive.

Yes. Vietnamese often leaves out the subject when it is obvious from context, especially in descriptions of weather, places, or general situations.

So this sentence is understood as something like:

• In the shade, it is very cool
• The shade is very cool
• It’s very pleasant under the shade

The exact English subject depends on context, but Vietnamese does not always need to say it.

Vietnamese often starts with the setting or topic, then gives the comment about it.

So this structure is:

• Dưới bóng râm = the setting / location
• rất mát mẻ = the comment about that setting

It is similar to English sentences like:

• In the shade, it’s very cool.
• Under the shade, it’s very pleasant.

This word order is very natural in Vietnamese.

Yes, both are possible, but they have slightly different feels.

• Ở dưới bóng râm makes the location a little more explicit
• Trong bóng râm emphasizes being inside the shaded area
• Dưới bóng râm is also natural, especially when the shade comes from something above, like a tree

So all of these can work depending on context.

They are close, but not always identical in feeling.

• bóng râm = shade, shadowed area
• bóng mát = cool shade, often with a more pleasant or restful feeling

In everyday speech, bóng mát is very common when people talk about sitting or resting in the shade.
Bóng râm is also correct and common, but can sound a bit more neutral.

Yes, it is understandable and natural, especially in context. But in everyday speech, native speakers might also say things like:

• Dưới bóng cây rất mát.
• Trong bóng râm mát lắm.
• Ngồi dưới bóng râm rất mát mẻ.

So your sentence is fine, but spoken Vietnamese often uses slightly different versions depending on the situation.