Одна чайка летела низко над водой.

Because одна agrees with чайка, and чайка is a feminine singular noun.

Russian один changes for gender:

• один — masculine
• одна — feminine
• одно — neuter

So with чайка (a seagull), you get одна чайка.

In this sentence, одна most naturally means one / a single seagull.

It can suggest one seagull or a single seagull, depending on context.

Russian has no articles (a / the), so чайка by itself could often mean a seagull or the seagull depending on context. Adding одна makes the idea of one single seagull more explicit.

Sometimes одна can also mean alone, but in this sentence the most natural reading is simply one seagull.

Чайка is in the nominative singular because it is the subject of the sentence — the thing doing the action.

The basic dictionary form is чайка. Since the sentence is about what the seagull did, nominative is exactly what you would expect.

Летела is the past tense, singular, feminine form of the verb лететь (to fly).

In Russian past tense, verbs agree with the subject in gender and number:

• летел — masculine singular
• летела — feminine singular
• летело — neuter singular
• летели — plural

Since чайка is feminine singular, the verb becomes летела.

Because Russian often expresses that idea with just the past tense form of an imperfective verb.

So летела can mean:

• flew
• was flying

The exact English translation depends on context. Russian does not need a separate word like was here.

These forms express different aspects or meanings:

• летела — she/it was flying, or flew in an ongoing sense; this describes the process
• полетела — she/it started flying or flew off; this focuses more on the beginning or completion
• летала — she/it used to fly / flew repeatedly / went flying on more than one occasion

In Одна чайка летела низко над водой, the speaker is simply describing the seagull in the middle of the action, so летела is the natural choice.

Because низко is an adverb, and it describes how the seagull was flying.

• низкий = low (adjective, used with nouns)
• низко = low / low down (adverb, used with verbs)

Here it modifies летела:

• летела низко = was flying low

Because над means above / over, and it normally takes the instrumental case.

So:

• над
  • instrumental = over / above something

The noun вода becomes водой in the instrumental singular.

So:

• над водой = over the water

Вода is a feminine noun ending in -а. In the instrumental singular, many nouns of this type change to -ой (or sometimes -ей, depending on spelling and stress patterns).

So:

• вода → водой

That is why after над, you see водой.

Yes. Russian word order is fairly flexible, because case endings show grammatical roles.

Одна чайка летела низко над водой is a neutral, natural order.

But Russian could also say things like:

• Низко над водой летела одна чайка.
• Над водой низко летела одна чайка.

These alternatives are grammatically fine, but they shift the focus or emphasis. Russian word order often reflects what information is new, important, or being highlighted.

Yes, literally над водой is something like above the water.

But in natural English, when something is moving through the air in that position, we often say over the water. So the translation uses the English phrasing that sounds most natural.

Russian над covers the idea of being positioned above / over something.

The usual stress is:

• одна́
• ча́йка
• лете́ла
• ни́зко
• над водо́й

A rough pronunciation guide:

• одна́ — ad-NA
• ча́йка — CHAI-ka
• лете́ла — lye-TYE-la
• ни́зко — NEES-ka
• над водо́й — nad va-DOY

Stress matters in Russian, so it is worth learning words together with their stressed syllables.