Bir koyun sürüsü gördük.

Turkish normally prefers subject/object + verb word order, and the verb usually comes at the end.

So:

• Bir koyun sürüsü = the object
• gördük = we saw

That makes the neutral word order:

• Bir koyun sürüsü gördük. = We saw a flock of sheep.

Other word orders are possible in Turkish, but they usually add emphasis or sound more marked.

Here bir means a / an.

So:

• bir koyun sürüsü = a flock of sheep

It can also literally mean one in other contexts, but here it works like an indefinite article.

This is a very common Turkish noun pattern: noun + noun.

• koyun = sheep
• sürü = herd, flock
• koyun sürüsü = sheep flock / flock of sheep

Turkish often expresses of relationships with a noun compound instead of using a separate word like English of.

So English:

• a flock of sheep

becomes Turkish:

• bir koyun sürüsü

The -sü is the 3rd person possessive ending, which appears in this kind of noun compound.

• sürü = flock
• koyun sürüsü = flock of sheep

This does not mean his/her flock here. In compounds like this, Turkish regularly adds that ending to the second noun.

You can think of it as part of the standard pattern:

• koyun sürüsü = flock of sheep
• araba kapısı = car door
• ev kapısı = house door

So in this sentence, -sü is grammatical structure, not personal possession.

That s is a buffer consonant.

The base word is:

• sürü

The ending here is a vowel-initial possessive ending, so Turkish inserts s to make pronunciation smoother:

• sürü + ü → sürüsü

This is very normal in Turkish.

Two things are happening here:

1. Koyun is a word that can refer to sheep in a general sense.
2. In Turkish noun compounds, the first noun is usually left in a bare form.

So:

• koyun sürüsü literally looks like sheep flock
• but it means a flock of sheep

This is normal Turkish usage. You do not need to make koyun plural here.

Because the object here is indefinite / non-specific.

In Turkish:

• Bir koyun sürüsü gördük. = We saw a flock of sheep.
  
  • no accusative ending

But:

• Bir koyun sürüsünü gördük. = We saw a certain / specific flock of sheep.
  
  • accusative -(y)ı / -(y)i / -(y)u / -(y)ü appears

So the version without accusative is the normal way to say we saw a flock of sheep in a general, indefinite sense.

Gördük can be broken down like this:

• gör- = see
• -dü- = past tense
• -k = we

So:

• gördük = we saw

This is a very useful pattern to learn.

Because of vowel harmony and sound changes.

The verb stem is:

• gör-

The past tense ending changes shape depending on the vowels around it:

• -dı / -di / -du / -dü

Since ö is a front rounded vowel, the matching form is -dü.

So:

• gör + dü + k → gördük

This is exactly the kind of vowel harmony Turkish uses all the time.

The past tense ending can appear as -dı/-di/-du/-dü or -tı/-ti/-tu/-tü.

Which one you get depends on the final sound of the verb stem.

Since gör- ends in r, which is a voiced sound, the ending uses d:

• gör-dü-k

If a stem ends in certain voiceless consonants, you often get t instead.

Yes, it can, depending on context.

In many sentences, bir simply means a/an:

• Bir koyun sürüsü gördük. = We saw a flock of sheep.

But if the speaker wants to emphasize number, it can also feel like:

• We saw one flock of sheep.

Usually context tells you which sense is stronger.

Yes, it is a very natural and common expression.

Some common animal-group expressions in Turkish are:

• koyun sürüsü = flock of sheep
• kuş sürüsü = flock of birds
• balık sürüsü = school of fish

So this is a useful pattern you can reuse with other animals.

Yes, but it usually does not need to.

• Bir koyun sürüsü gördük. = We saw a flock of sheep.
• Biz bir koyun sürüsü gördük. = We saw a flock of sheep.

Turkish often leaves out subject pronouns because the verb ending already shows the person:

• -k in gördük already tells you it means we

So biz is optional and usually added only for emphasis or contrast.