Nous étions en train de désherber quand nous avons vu une racine sous le vieux rosier.

Être en train de + infinitive means to be in the middle of doing something or to be busy doing something.

So:

nous étions en train de désherber
= we were weeding / we were in the middle of weeding

It adds emphasis that the action was ongoing at that moment.

Étions is the imparfait form of être for nous.

In this sentence, French uses the imparfait because the weeding was an ongoing background action in the past. Avons été would be passé composé, which usually describes a completed event, and that would not fit the same idea here.

So:

• nous étions en train de désherber = ongoing action
• nous avons vu = completed event that happened during that action

After en train de, French uses an infinitive.

So the pattern is:

être en train de + infinitive

Examples:

• je suis en train de manger
• ils sont en train de travailler
• nous étions en train de désherber

That is why you do not say nous étions en train désherbions or anything similar.

Yes. Nous désherbions quand nous avons vu... is grammatically correct.

The difference is nuance:

• Nous désherbions = we were weeding
• Nous étions en train de désherber = we were in the middle of weeding

The version with en train de is a little more explicit and can sound more vivid or emphatic.

This is a very common French pattern:

• imparfait for the ongoing background action
• passé composé for the event that happened during it

Here:

• nous étions en train de désherber = the ongoing situation
• nous avons vu une racine = the specific thing that happened

English often does the same with was/were doing + did/saw:

We were weeding when we saw a root...

Because we saw a root is treated as a single completed event.

Nous avons vu means we saw in the sense of a finished moment of perception.

If you said nous voyions, it would suggest something more ongoing or habitual, such as we could see or we used to see, depending on context. That is not the idea here.

Here quand means when.

It introduces the moment when the second action happened:

We were weeding when we saw a root...

It does not automatically force one tense, but in this kind of sentence it often connects:

• an ongoing action in the imparfait
• with a completed event in the passé composé

So quand works very naturally with this tense contrast.

Because French normally requires an explicit subject pronoun with each conjugated verb.

So you say:

Nous étions ... quand nous avons vu ...

Even though English can sometimes avoid repetition more easily, French usually keeps the subject pronoun before each verb.

Désherber means to weed, to remove weeds, or to clear weeds from an area.

It is built from:

• herbe = grass / herb / plant growth
• the prefix dés-, which often suggests removal or undoing

So désherber is literally the idea of removing unwanted plant growth.

Une racine means a root.

It uses une because racine is a feminine noun in French.

So:

• une racine
• la racine
• des racines

There is no special reason it is feminine; that is simply the grammatical gender of the noun.

Un rosier is a rosebush or rose plant, not a single rose flower.

So:

• une rose = a rose
• un rosier = a rosebush

That is why sous le vieux rosier means under the old rosebush.

In French, many adjectives usually come after the noun, but some common ones often come before it. Vieux is one of those.

So:

• un vieux rosier = an old rosebush

This is normal adjective placement. Vieux often comes before the noun, like:

• un vieux livre
• une vieille maison
• de vieux arbres

Le means the, so sous le vieux rosier refers to a specific old rosebush, probably one already known in the situation.

If you said sous un vieux rosier, it would mean under an old rosebush, with a less specific sense.

Both are possible in French, but they do not mean exactly the same thing.

Yes. Quand and lorsque can both mean when.

So you could say:

Nous étions en train de désherber lorsque nous avons vu une racine sous le vieux rosier.

That would be correct. Lorsque is often a little more formal or literary, while quand is very common in everyday French.

A few points may stand out:

• étions sounds roughly like ay-tyon
• désherber has a z sound at the start: day-zher-bay
• rosier sounds roughly like ro-zee-ay
• vu has the French u sound, which is different from English oo

A rough pronunciation of the whole sentence is:

Noo zay-tyon ahn tran duh day-zher-bay, kahn noo zah-von vyoo oon ra-seen soo luh vyuh ro-zee-ay.

That is only an approximation, but it can help you get started.

Yes, it sounds natural.

It is a good example of a very common French storytelling pattern:

• background action in the imparfait
• sudden or completed event in the passé composé

A native speaker might also simply say Nous désherbions quand nous avons vu..., but the sentence as given is perfectly normal and clear.