Судя по номеру рейса, порт назначения изменился.

Судя по means judging by, based on, or going by.

So:

• Судя по номеру рейса = Judging by the flight number
• The speaker is making an inference from evidence, not stating a directly observed fact.

A natural English sense is:

• Judging by the flight number, the destination port has changed.
• Based on the flight number, it looks like the destination has changed.

Grammatically, судя comes from судить (to judge), but in modern Russian судя по is often treated as a fixed expression.

Because по here requires the dative case.

• номер = nominative singular
• номеру = dative singular

So:

• по номеру = by / according to the number

This is very common with по in the sense of according to or judging from:

• по расписанию = according to the schedule
• по документам = according to the documents
• по виду = judging by the appearance

So судя по номеру рейса literally works like judging according to the flight number.

Because рейса is in the genitive singular after номер.

The pattern is:

• номер чего? = the number of what?
• номер рейса = the flight number

This is a standard Russian noun-to-noun relationship:

• номер поезда = train number
• номер телефона = phone number
• номер комнаты = room number

So:

• по номеру рейса = by the flight number

Назначения is genitive singular.

The phrase порт назначения literally means:

• порт = port
• назначение = destination / assignment / purpose, depending on context
• порт назначения = port of destination or more naturally destination port

This is another very common Russian structure:

• центр города = city center
• план поездки = trip plan
• место встречи = meeting place

So назначения depends on порт and tells you what kind of port it is.

Because the verb agrees with the subject порт, which is masculine singular.

• порт = masculine
• therefore past tense = изменился

Compare:

• порт изменился = the port changed
• дата изменилась = the date changed
• расписание изменилось = the schedule changed

Even though назначения is nearby, it is not the main noun. The whole subject is порт назначения, and the head noun is порт.

Изменился is from измениться, the perfective verb, and it presents the change as a completed event or result.

So it means something like:

• has changed
• changed
• turned out to be different now

If you used the imperfective менялся, it would suggest a process, repetition, or ongoing change, which is not the idea here.

Compare:

• Порт назначения изменился. = The destination port changed / has changed.
• Порт назначения менялся несколько раз. = The destination port changed several times.

In this sentence, the perfective is the natural choice because the speaker is talking about the resulting new destination.

It suggests an inference, not absolute certainty.

Because of судя по, the speaker is saying:

• As far as I can tell from the flight number...
• It seems that...
• Apparently...

So the nuance is not I know for sure the destination changed, but rather:

• The evidence of the flight number suggests that the destination changed.

That nuance is important.

Because судя по номеру рейса is an introductory adverbial phrase, and it is normally separated by a comma.

Structure:

• Судя по номеру рейса, = introductory phrase
• порт назначения изменился. = main clause

This is similar to English punctuation in sentences like:

• Judging by the map, we’re in the wrong place.

Russian often sets off such introductory expressions with commas.

Yes. Russian word order is flexible.

The neutral version here is:

• Судя по номеру рейса, порт назначения изменился.

But other orders are possible, depending on emphasis:

• Порт назначения, судя по номеру рейса, изменился.
• Порт назначения изменился, судя по номеру рейса.

These alternatives are understandable, but the original version is the most natural if you want to begin with the evidence: judging by the flight number...

Not necessarily. It literally means port of destination, and in the right context it can be quite literal, especially with shipping or maritime travel.

But depending on the broader context, Russian sometimes uses transport terminology in ways that may sound slightly broader in translation. If the sentence is about aviation, a learner might expect аэропорт назначения (destination airport) instead.

So the exact English translation may depend on context:

• destination port
• destination airport
• more generally, destination

If your source gives a meaning already, follow that context.

Yes, but it depends on style.

Possible translations:

• Judging by the flight number
• Based on the flight number
• According to the flight number

The first two are usually the most natural in English, because the Russian phrase implies inference from evidence.

According to the flight number is understandable, but it can sound slightly more mechanical or literal.

Historically, судя is a gerund-like adverbial form from судить (to judge).

In practice, learners should usually treat судя по as a fixed expression meaning:

• judging by
• based on

Important points:

• судя does not change for gender, number, or person
• it is commonly followed by по + dative
• it introduces the basis for a conclusion

So for learning purposes, the safest approach is:

• memorize судя по + dative

For example:

• Судя по голосу, он устал. = Judging by his voice, he is tired.
• Судя по всему, они опоздают. = Judging by everything / apparently, they will be late.

Russian has no articles.

So:

• номер рейса can mean the flight number or a flight number
• порт назначения can mean the destination port or a destination port

The exact meaning comes from context.

In this sentence, English usually wants the:

• Judging by the flight number, the destination port has changed.

That does not mean Russian has a hidden word for the here; it simply leaves definiteness to context.

Yes — several useful patterns:

1. судя по + dative
   = judging by / based on

   • судя по расписанию
   • судя по погоде
   • судя по выражению лица

2. номер + genitive
   = number of ...

   • номер рейса
   • номер поезда
   • номер телефона

3. [noun] + genitive for of ... relationships

   • порт назначения
   • центр города
   • цель поездки

4. Past-tense verb agreement with the main noun

   • порт изменился
   • дата изменилась
   • расписание изменилось

Those are all very productive patterns in Russian.