Encontrei uma pista junto à bicicleta.

Encontrei is the 1st person singular form of encontrar in the pretérito perfeito simples — in English terms, the simple past / preterite.

So:

• encontrei = I found
• it refers to a completed action in the past

Examples:

• Encontrei uma pista. = I found a clue.
• Encontrei as chaves. = I found the keys.

Because Portuguese often drops the subject pronoun when it is clear from the verb ending.

• Encontrei already tells you the subject is I
• so Eu encontrei uma pista junto à bicicleta is possible, but eu is not necessary

Using eu usually adds emphasis, contrast, or clarity:

• Eu encontrei a pista, não ele. = I found the clue, not him.

Without emphasis, the version in your sentence is the most natural.

Because in Portuguese, a singular countable noun usually needs an article when you mean a/an.

So:

• uma pista = a clue / a track
• just pista on its own would sound incomplete in a normal sentence like this

Portuguese uses articles more regularly than English in many situations. Here, uma introduces a new, unspecified thing.

Because à is a contraction of:

• a (the preposition)
• • a (the feminine singular definite article)

So:

• junto a a bicicleta becomes junto à bicicleta

This is exactly why the grave accent appears:

• à = a + a

Important: the accent in à does not mark stress like other accents often do. Its main job here is to show the contraction.

Here junto à means something like:

• next to
• beside
• right by
• close to

So it expresses physical proximity.

It is slightly more formal or neutral in tone than some everyday alternatives. It does not mean with in the sense of together with here; it means near.

Yes.

You may hear several options in European Portuguese:

• junto à bicicleta
• junto da bicicleta
• ao lado da bicicleta
• perto da bicicleta

Roughly speaking:

• junto à / junto da = close to / right by
• ao lado da = beside / at the side of
• perto da = near, more general and less exact

So Encontrei uma pista ao lado da bicicleta is also natural. The sentence you were given is perfectly correct; it is just one of several ways to express the idea.

Because bicicleta is a feminine noun in Portuguese.

That is why it takes feminine forms like:

• a bicicleta
• uma bicicleta
• à bicicleta

This is grammatical gender, not biological gender. Objects in Portuguese are assigned masculine or feminine gender, and the article has to match the noun.

Yes. The original order is the most neutral:

• Encontrei uma pista junto à bicicleta.

But you can move the location phrase for emphasis:

• Junto à bicicleta, encontrei uma pista.

That version puts more focus on where the clue was found. It is grammatical, but it sounds a bit more marked or literary than the basic order.

No. Pista has several meanings in Portuguese, depending on context.

Common meanings include:

• clue
• track
• runway
• lane
• sometimes dance floor in certain contexts

So the word is not limited to just one meaning. In your sentence, the intended meaning is already clear from the context you were given, but learners often notice that pista is a flexible word.

A rough learner-friendly pronunciation is:

en-con-TRAI oo-muh PEESH-tuh ZHOON-too ah bee-see-KLEH-tuh

A few useful pronunciation points for European Portuguese:

• encontrei ends with a sound like -trai
• pista: the s before t sounds like sh in Portugal, so PEESH-tuh
• junto: the j sounds like the s in measure
• à is very short and unstressed
• bicicleta has the stress on -kle-

European Portuguese also tends to reduce unstressed vowels more than English speakers expect, so in real speech the sentence may sound more compressed than the spelling suggests.