No cubrió la base, sino que llegó al borde.

They are both in the preterite tense: cubrió = he/she/it covered, and llegó = he/she/it arrived/reached.

Spanish uses the preterite for actions seen as completed events. In this sentence, the speaker is talking about two finished actions:

• No cubrió la base = he/she/it did not cover the base
• sino que llegó al borde = but instead he/she/it reached the edge

If the idea were more descriptive, ongoing, or habitual, Spanish might use the imperfect instead.

Spanish often leaves out subject pronouns because the verb ending already tells you the person and number.

Here:

• cubrió
• llegó

both clearly mean he/she/it.

So Spanish usually does not say él or ella unless the speaker wants to:

• emphasize the subject,
• contrast it with someone else,
• or avoid confusion.

A full version like Él no cubrió la base, sino que llegó al borde is possible, but it sounds more emphatic.

Spanish uses no ... sino ... or no ... sino que ... to correct a previous idea: not X, but rather Y.

So:

• No cubrió la base, sino que llegó al borde
  means something like
• He didn’t cover the base; rather, he reached the edge

This is different from pero, which usually just means but in a more general contrast:

• Intentó cubrir la base, pero llegó tarde
  = He tried to cover the base, but he arrived late.

Here, the second part is not just a contrast: it replaces the first idea. That is why sino que is the natural choice.

Use sino by itself when what follows is just a noun, adjective, or phrase, not a new conjugated verb.

Examples:

• No era rojo, sino azul.
• No vino Juan, sino María.

Use sino que when what follows is a full clause with a conjugated verb.

Here, the second part is:

• llegó al borde

That is a full clause with the verb llegó, so Spanish uses sino que:

• No cubrió la base, sino que llegó al borde.

Because a + el contracts to al in Spanish.

So:

• a el borde → al borde

This contraction is mandatory.

The only common exception is when el is part of a proper name, for example:

• a El Escorial

But with ordinary nouns, you must contract:

• al borde
• al final
• al suelo

The verb llegar normally takes the preposition a before a destination.

So Spanish says:

• llegar a casa
• llegar al centro
• llegar al borde

That is why the sentence has:

• llegó al borde

Even though English might say reached the edge without a preposition, Spanish normally uses llegar a.

No, cubrió is a regular preterite form of cubrir.

The pattern is:

• cubrí
• cubriste
• cubrió
• cubrimos
• cubristeis
• cubrieron

So cubrió is just the normal third-person singular preterite.

A learner may expect something stranger because some Spanish verbs do change in the preterite, but cubrir does not here.

The accent marks show the correct stress and are part of the standard written forms.

• cubrió
• llegó

In the preterite, many third-person singular forms end in -ó, and that written accent is normal:

• habló
• comió
• vivió
• llegó
• cubrió

So the accents are not optional; they are just the correct spelling.

Because nouns in Spanish have grammatical gender.

• la base is feminine
• el borde is masculine

The article must match the noun:

• la for feminine singular
• el for masculine singular

This is mostly something you have to learn with each noun. The meaning does not have to be literally feminine or masculine.

Yes, but it would change the meaning.

Your sentence uses the preterite:

• No cubrió la base, sino que llegó al borde.

This presents the actions as completed events.

If you used the imperfect:

• No cubría la base, sino que llegaba al borde.

it would sound more like a description of what was happening, what used to happen, or what was in progress, rather than a single completed event.

So the original sentence sounds like a specific finished action, which is why the preterite is the natural choice.

Yes, normally it does.

In:

• No cubrió la base, sino que llegó al borde

the natural reading is that the same person or thing both:

• did not cover the base
• and did reach the edge

Spanish does not need to repeat the subject if it is clear from context. If the speaker wanted to make the change of subject explicit, they would usually say so more clearly.