La semilla germinó; trasplanté la planta, pero luego se pudrió.

Because the sentence tells a sequence of completed events:

• the seed sprouted
• I transplanted the plant
• then it rotted

In Spanish, the preterite is the normal tense for single, finished actions in a story.

If you used the imperfect instead, it would sound more like background description, an ongoing action, or a habitual action, not a clear chain of events.

They are:

• germinó ← germinar
• trasplanté ← trasplantar
• se pudrió ← pudrirse

More specifically:

• germinó = third person singular preterite
• trasplanté = first person singular preterite
• se pudrió = third person singular preterite of the pronominal verb pudrirse

Because Spanish often leaves out subject pronouns when they are not needed.

The ending -é in trasplanté already tells you the subject is I. So yo would usually be unnecessary unless you wanted emphasis or contrast.

This is very normal in Spanish:

• Trasplanté la planta = I transplanted the plant

Here, pudrirse means to rot, to go rotten, or to become rotten.

The se is part of the verb in this meaning. It shows that the subject itself undergoes the change:

• La planta se pudrió = the plant rotted

Without se, pudrir is often used transitively, meaning something causes something else to rot:

• El exceso de agua pudrió la planta = too much water rotted the plant

So in your sentence, se pudrió is the natural choice.

The understood subject is la planta.

Spanish often omits a repeated subject when it is clear from context. In this sentence, the most natural interpretation is:

• La semilla germinó; trasplanté la planta, pero luego la planta se pudrió.

Since repeating la planta is not necessary, Spanish leaves it out.

Spanish normally uses articles where English sometimes would not.

Here, la semilla and la planta refer to specific things in the story:

• the seed we are talking about
• the plant that came from it

So the definite article sounds natural.

Also, once something has been introduced or is clearly identifiable in context, Spanish often uses el/la.

Both are possible.

• Trasplanté la planta sounds explicit and clear.
• La trasplanté would also be natural if the reference is already obvious.

Using la planta here helps the listener follow the change from seed to plant. It makes the sentence especially clear for a narrative sequence.

Here, luego means then, afterwards, or later.

Yes, después would also work in this sentence:

• ..., pero después se pudrió.

In Spain, luego is very common in narration to move the story forward. It sounds very natural here.

The semicolon separates two closely connected parts of the sentence:

• La semilla germinó
• trasplanté la planta, pero luego se pudrió

It is stronger than a comma but weaker than a full stop. It works well here because the second part is already a bit longer and contains its own comma before pero.

A full stop would also be possible:

• La semilla germinó. Trasplanté la planta, pero luego se pudrió.

The accent marks show the correct stress and, in some cases, help distinguish forms.

For example:

• germinó = he/she/it germinated

• germino = I germinate

• trasplanté = I transplanted

• trasplante could be a noun or a different verbal form

• pudrió = he/she/it rotted

These accented preterite forms are very common in Spanish:

• first person singular of -ar verbs: -é
• third person singular of -ar verbs: -ó
• many third person singular forms like -ió also carry an accent

Yes, that is grammatically possible, but it sounds a little less polished.

Using a semicolon gives the sentence a cleaner structure because the second part already contains pero. It helps separate the first completed event from the more complex second part.

So:

• La semilla germinó; trasplanté la planta, pero luego se pudrió. = more elegant written style
• La semilla germinó y trasplanté la planta, pero luego se pudrió. = possible, but a bit heavier

It refers to the plant.

Even though both semilla and planta are feminine singular, the context makes it clear. First the seed sprouts, then it becomes a plant, then that plant is transplanted, and later that plant rots.

So the logic of the situation tells you the final subject is la planta.