ربما يقرر صديقي أن يذهب بالحافلة إذا كان الطقس باردا.

ربما means perhaps, maybe, or sometimes probably, depending on context.

In this sentence, it signals possibility, so the idea is something like:

• Maybe my friend will decide...
• My friend might decide...

It often appears at the beginning of the sentence, as it does here, to set the tone of uncertainty right away.

Arabic often prefers verb-first word order, especially in neutral statements.

So:

• يقرر صديقي = my friend decides / might decide

This is completely normal in Modern Standard Arabic.

You could also say:

• صديقي يقرر...

but that would sound a little more like you are emphasizing my friend.

So the order in your sentence is standard and natural:

• ربما يقرر صديقي...

In Arabic, the present tense often covers both:

• present meaning
• future meaning

The future meaning usually becomes clear from context.

Here, ربما plus the whole situation makes it natural to understand يقرر as something that may happen in the future:

• ربما يقرر = he may decide / might decide

Arabic does not always need سـ or سوف to express the future.

صديقي means my friend.

Breakdown:

• صديق = friend
• ـي = my

So:

• صديقي = my friend

This is a very common possessive pattern in Arabic. The noun is connected directly to the possessive suffix.

Other examples:

• كتابي = my book
• بيتي = my house
• أستاذي = my teacher

أن is used before a present-tense verb to introduce something like to ... or that ... after certain verbs.

Here:

• يقرر أن يذهب = he decides to go

So أن links decide with the action being decided.

Very common patterns are:

• أريد أن أذهب = I want to go
• أحاول أن أتعلم = I try to learn
• يقرر أن يذهب = he decides to go

Yes. After أن, the present-tense verb is normally in the subjunctive.

So in full vocalization, it would be:

• يذهبَ

not:

• يذهبُ

But in normal unvowelled Arabic writing, both are usually written the same way:

• يذهب

So the grammar is there, even if you do not see it in plain text.

The subject of يذهب is understood to be the same person: صديقي.

So:

• يقرر صديقي أن يذهب
  = my friend decides to go

Arabic often leaves the subject unstated when it is already clear from the verb form or context.

Here يذهب is he goes, so it naturally refers back to my friend.

The preposition بـ often expresses means or instrument, similar to English by.

So:

• بالحافلة = by bus
• literally: with / by means of the bus

Breakdown:

• بـ = by / with
• الحافلة = the bus
• بالحافلة = by bus

This is a very common pattern with transportation:

• بالسيارة = by car
• بالقطار = by train
• بالطائرة = by plane

Arabic often uses the definite article in expressions where English does not.

So بالحافلة literally contains the bus, but the natural English translation is still just:

• by bus

This is normal and does not necessarily mean a specific bus in the way English the bus might.

This is the if-clause.

Breakdown:

• إذا = if
• كان = was / is in this kind of structure
• الطقس = the weather
• باردا = cold

Together:

• إذا كان الطقس باردا = if the weather is cold

So the full sentence structure is:

• Perhaps my friend may decide to go by bus
• if the weather is cold

This is a very common Arabic pattern.

After إذا, Arabic often uses a past-tense form such as كان, even when the meaning is present or future in English.

So:

• إذا كان الطقس باردا
  literally looks like if the weather was cold
• but actually means
  if the weather is cold

This is normal Arabic conditional style, not a mistake.

باردا is the predicate of كان.

With كان, the pattern is:

• the subject stays nominative
• the predicate becomes accusative

So:

• الطقسُ = the weather (subject of كان)
• باردًا = cold (predicate of كان)

In ordinary writing without full vowel marks, you often see it simply as:

• باردا

But in fully vocalized form, it is:

• باردًا

Because here it is an indefinite predicate, meaning simply cold, not the cold one.

Arabic often leaves predicate adjectives indefinite in sentences like this.

So:

• الطقس بارد = the weather is cold
• كان الطقس باردا = the weather was / is cold

Using البارد here would usually not be the normal choice.

Here is a simple breakdown:

• ربما = particle/adverb of possibility
• يقرر = verb, he decides / may decide
• صديقي = subject, my friend
• أن = particle introducing the following verb phrase
• يذهب = verb, he goes / to go
• بالحافلة = prepositional phrase, by bus
• إذا = if
• كان = verb used in the conditional clause
• الطقس = subject of كان
• باردا = predicate of كان

So the sentence is built very logically:

• possibility
• main action
• person doing it
• action decided upon
• means of transport
• condition

A fully vocalized version would be:

رُبَّما يُقَرِّرُ صَديقي أَنْ يَذْهَبَ بِالحافِلَةِ إِذا كانَ الطَّقْسُ بارِدًا

This helps you see some important grammar:

• يُقَرِّرُ → indicative present
• يَذْهَبَ → subjunctive after أن
• الطَّقْسُ → nominative
• بارِدًا → accusative after كان

In normal texts, many of these endings are not written, so learners often have to infer them from grammar.