Ao fim da tarde, nós caminhamos até à margem do lago.

It is a very common time expression meaning in the late afternoon or towards evening.

Literally, it is something like at the end of the afternoon, but in natural English you would usually not translate it word for word.

A quick breakdown:

• ao = a + o
• fim = end
• da tarde = of the afternoon

So the whole phrase works as a set expression for a time of day.

Both can exist, but ao fim da tarde is a very common idiomatic way to say late in the afternoon / towards evening.

A rough nuance is:

• ao fim da tarde = a standard time expression, very natural for late afternoon
• no fim da tarde = can also be correct, but may feel a bit more like at the end of the afternoon as a specific point

In everyday use, the difference is often small, but ao fim da tarde is especially common in this kind of sentence.

Usually, no.

Portuguese often drops subject pronouns because the verb ending already shows who the subject is. So:

• Nós caminhámos até à margem do lago.
• Caminhámos até à margem do lago.

Both are correct.

In European Portuguese, leaving out nós is very common unless you want:

• emphasis
• contrast
• extra clarity

So nós is possible, but not required.

Yes — in standard European Portuguese, if the meaning is we walked, the usual spelling is caminhámos.

This matters because:

• caminhamos = usually we walk / we are walking (present)
• caminhámos = we walked (simple past)

This spelling distinction is typical of European Portuguese. In Brazilian Portuguese, both are normally written caminhamos, and context tells you which tense is meant.

So for Portugal Portuguese, if the intended meaning is past, many learners would expect:

• Ao fim da tarde, nós caminhámos até à margem do lago.

Because até emphasizes reaching the endpoint.

Compare:

• caminhar até à margem = walk as far as the shore/bank; you reached it
• caminhar para a margem = walk toward the shore/bank; direction is the main idea, not necessarily arrival

So até is a good choice when the sentence wants to show the movement continued all the way to that place.

Because margem is a feminine noun and here it comes with the definite article a.

So:

• a margem = the bank / the shore
• after até, European Portuguese very often contracts this as até à

So:

• até à margem = up to the bank/shore

You may sometimes also see até a margem, but até à margem is very common and very natural in Portugal Portuguese.

Here margem means the edge, bank, or shore of the lake.

For bodies of water like rivers and lakes, margem is a very normal word. It refers to the side or edge of the water.

So:

• a margem do lago = the shore/bank of the lake

A related word is beira, which can also mean edge or side, but margem is a very solid, standard choice for a lake or river.

Because Portuguese usually uses the definite article with a specific noun like the lake.

So:

• o lago = the lake
• de + o = do

That gives:

• do lago = of the lake

If you meant of a lake, you would say:

• de um lago

Yes.

Portuguese is fairly flexible with word order, especially with time expressions. For example, you could also say:

• Nós caminhámos até à margem do lago ao fim da tarde.
• Caminhámos até à margem do lago ao fim da tarde.

Starting with Ao fim da tarde is very natural because it sets the scene first.

The comma is also natural here because Ao fim da tarde is an introductory time phrase.