No sé si ganaremos el campeonato, pero este rival nos obliga a jugar mejor.

Because si here means whether / if.

• No sé si ganaremos... = I don’t know whether we’ll win...
• Que would not work in this structure.

A useful pattern is:

• No sé si + clause = I don’t know if / whether + clause

For example:

• No sé si vendrá = I don’t know if he/she will come
• No sé si tienen tiempo = I don’t know if they have time

So in this sentence, si introduces an indirect yes/no question.

Because the speaker is talking about something uncertain in the future: whether we will win the championship.

• ganamos = we win / we are winning
• ganaremos = we will win

The sentence is about a result that has not happened yet, so the future is natural here.

Also, after No sé si..., Spanish often uses the normal future tense when talking about a real future possibility:

• No sé si iremos = I don’t know if we’ll go
• No sé si llegará a tiempo = I don’t know if he/she will arrive on time

Because si in this kind of sentence usually takes the indicative, not the subjunctive.

Here, the speaker is simply unsure about a fact:

• No sé si ganaremos el campeonato

This is an indirect question about whether something will happen. Spanish normally uses the indicative here, including the future indicative.

Compare:

• No sé si viene = I don’t know if he/she is coming
• No sé si vendrá = I don’t know if he/she will come

The subjunctive is not used just because there is uncertainty. That is a very common difficulty for English speakers.

Ganaremos is:

• the verb ganar = to win
• future indicative
• first person plural
• meaning we will win

The future endings in Spanish are added to the infinitive:

• ganar
  • -emos = ganaremos

Other forms:

• ganaré = I will win
• ganarás = you will win
• ganará = he/she will win
• ganaremos = we will win
• ganaréis = you all will win
• ganarán = they will win

Spanish often uses the definite article more than English does.

So:

• ganar el campeonato = to win the championship

Even where English might sometimes say win championship in certain informal contexts, standard Spanish normally wants the article here.

This is very common with nouns referring to titles, sports competitions, body parts, days, and general categories.

Rival means opponent or rival.

In sports contexts, rival is very common in Spanish, often more common than oponente in everyday use.

• este rival = this opponent / this rival

The demonstrative este points to something close to the speaker’s perspective, either physically or mentally.

Compare:

• este rival = this rival
• ese rival = that rival
• aquel rival = that rival over there / that more distant rival

So este rival sounds like this particular opponent we’re talking about.

In this sentence, it is masculine because we have este rival.

But rival can refer to a man or a woman. The article or adjective usually shows the gender:

• este rival = this male rival / this rival (masculine)
• esta rival = this female rival

The noun itself often stays the same in form.

Obligar a means to force someone to or to make someone do something.

Here:

• nos = us
• obliga = forces / makes
• a jugar = to play

So:

• este rival nos obliga a jugar mejor
• literally: this rival forces us to play better

This structure is very common:

• obligar a alguien a + infinitive

Examples:

• Me obliga a estudiar = It forces me to study
• Nos obligan a esperar = They make us wait
• La situación lo obliga a cambiar = The situation forces him to change

Because the verb obligar is followed by the pattern:

• obligar a alguien a + infinitive

So the first a is part of the indirect object structure with the person, and the second a introduces the infinitive.

In this sentence, the person is replaced by the pronoun nos, so you only see the second a clearly:

• nos obliga a jugar

Compare with a full noun:

• Este rival obliga al equipo a jugar mejor

That makes the pattern easier to see:

• obliga al equipo = forces the team
• a jugar = to play

Because object pronouns usually go before a conjugated verb in Spanish.

So:

• nos obliga = forces us

This is the normal order with a finite verb.

Compare:

• me ayuda = helps me
• te llama = calls you
• nos obliga = forces us

If the verb were an infinitive or a positive command, the pronoun could attach to the end:

• obligarnos = to force us
• ayúdame = help me

But here, since obliga is conjugated, nos goes before it.

Because mejor already means better.

So:

• jugar bien = to play well
• jugar mejor = to play better

You do not say más mejor, just as in English you do not say more better.

Also, here mejor is functioning adverbially: it describes how we play.

No. Here it means better, not best.

• mejor can mean better or best depending on context
• in this sentence, it is clearly comparative: to play better

Why? Because the idea is that this opponent pushes us to improve our level, not necessarily to play at the absolute best level possible.

If you wanted the best, you would usually need a different structure, such as:

• jugar lo mejor posible = to play as well as possible

Pero means but and introduces a contrast.

The contrast is:

• We don’t know if we’ll win the championship
• but
• this opponent makes us play better

So the sentence contrasts uncertainty about the final result with a clear statement about the effect of this opponent.

Yes. That would also be natural.

• nos obliga a jugar mejor = forces us to play better
• nos hace jugar mejor = makes us play better

The difference is one of nuance:

• obliga a sounds stronger, like the opponent compels us to raise our level
• hace is a bit more neutral

In sports commentary, obliga a jugar mejor is very natural because it suggests the rival is so strong that we have to improve.

Both are possible, but they are slightly different in feel.

• ganaremos = we will win
• vamos a ganar = we are going to win

In many cases, both express future meaning. The simple future ganaremos is very natural after No sé si... when talking about an uncertain future result.

So:

• No sé si ganaremos el campeonato sounds completely normal
• No sé si vamos a ganar el campeonato is also possible and sounds a bit more like a planned or expected future

In this sentence, the simple future is concise and elegant.

A close, word-for-word version would be:

• I don’t know if we will win the championship, but this opponent forces us to play better.

A more natural English version might be:

• I don’t know if we’ll win the championship, but this opponent makes us play better.
• I don’t know if we’ll win the championship, but this rival forces us to play better.

So the Spanish structure is very close to English here, which makes it a useful sentence for learners.