syokutaku no ue ni hana wo kazaru to, heya ga sukosi akaruku miemasu.

Because に marks the place where something ends up or is placed.

In 花を飾る, the flowers are being put/displayed on the table, so 食卓の上に means on top of the dining table as the target location.

• に = destination / resulting location
• で = the place where an action happens

So:

• 食卓の上に花を飾る = decorate the top of the table with flowers / place flowers on the table
• 食卓の上で would sound more like the table is the location where some action is performed, which is not the main idea here

Here, と means when or if, and it expresses a natural result.

So 花を飾ると、部屋が少し明るく見えます means something like:

• When you decorate the table with flowers, the room looks a little brighter
• If you put flowers on the table, the room appears a little brighter

This と often suggests a regular, expected, or automatic result rather than a one-time personal decision.

Because the conditional と attaches to the plain form of a verb.

So:

• 飾る + と = when/if (someone) decorates
• 行く + と = when/if (someone) goes
• 見る + と = when/if (someone) sees

This is just the normal grammar pattern.

が marks 部屋 as the subject of 見えます.

The sentence is focusing on what appears brighter:

• 部屋が少し明るく見えます = the room looks a little brighter

If you used は, it would sound more like you were setting the room up as the topic, possibly with some contrast:

• 部屋は少し明るく見えます = as for the room, it looks a little brighter

That is possible in some contexts, but が is more neutral and natural here.

Because 明るい is an い-adjective, and before a verb like 見える, it changes to its adverb-like form:

• 明るい → 明るく

So:

• 明るい部屋 = a bright room
• 明るく見える = to look bright

In this sentence, 明るく modifies 見えます, so it means looks bright.

They are very different verbs.

• 見る = to look at / to watch
• 見える = to be visible / to look / to appear

So:

• 部屋を見ます = I look at the room
• 部屋が明るく見えます = the room looks bright

In this sentence, the meaning is not that someone is actively looking at the room, but that the room appears brighter.

Not necessarily. It means it looks brighter.

That is an important nuance. The sentence describes appearance or impression, not an objective measurement of light.

So this could mean:

• the flowers make the room seem more cheerful
• the room gives a brighter visual impression

If you wanted to say the room actually becomes brighter, you would probably use a different expression.

Because 花 is the direct object of 飾る.

• 花を飾る = decorate with flowers / display flowers

The action is being done to the flowers in the sense that they are what is arranged or displayed.

So:

• 花を = flowers as the object
• 飾る = to decorate / to adorn / to display

食卓 means dining table or table used for meals.

It is similar to テーブル, but the nuance is a bit more specific:

• 食卓 = dining table, meal table
• テーブル = table in general, often just the loanword table

So 食卓の上 specifically suggests the top of the dining table.

少し means a little or slightly, and here it modifies the idea of 明るく見えます.

So the meaning is:

• looks a little brighter
• appears slightly bright / brighter

In natural English, a little brighter is the best fit.

Yes, the subject is omitted because Japanese often leaves it unstated when it is clear or unimportant.

So 花を飾ると could mean:

• when you decorate the table with flowers
• when we put flowers on the table
• when one places flowers on the table

Japanese does not need to specify the subject here unless it matters.

Yes. That is one of the natural effects of と.

It often sounds like:

• Whenever you put flowers on the table, the room looks a little brighter
• Putting flowers on the table makes the room look a little brighter

So the sentence can feel like a general observation, not just one single occasion.

Yes, but the nuance changes.

• 飾ると = when/if you decorate, then this naturally happens
• 飾れば = if you decorate, then this will happen / would happen

So:

• 花を飾ると、部屋が少し明るく見えます sounds like a general, natural result
• 花を飾れば、部屋が少し明るく見えます sounds a bit more like a condition or suggestion

Both are grammatical, but と fits the idea of a usual effect especially well.

It breaks down like this:

• 食卓の上に = on the dining table
• 花を = flowers
• 飾ると = when/if you decorate with them
• 部屋が = the room
• 少し = a little
• 明るく = brightly / bright in appearance
• 見えます = looks / appears

So the overall structure is:

When flowers are placed on the dining table, the room looks a little brighter.