Vi un pulgón en el tallo.

Vi is the first-person singular preterite form of ver (to see).

• ver = to see
• vi = I saw

Spanish often uses the preterite for a completed action in the past, so Vi un pulgón means I saw an aphid.

A quick mini-conjugation of ver in the preterite:

• yo vi = I saw
• tú viste = you saw
• él/ella vio = he/she saw
• nosotros vimos = we saw
• vosotros visteis = you all saw
• ellos vieron = they saw

Because Spanish often omits subject pronouns when the verb already makes the subject clear.

• Vi un pulgón en el tallo = I saw an aphid on the stem
• Yo vi un pulgón en el tallo = also correct, but yo is usually only added for emphasis, contrast, or clarity

The verb vi already tells you the subject is I.

Un pulgón means an aphid.

A few useful points:

• pulgón is a masculine singular noun
• so it uses un, not una
• plural: pulgones = aphids

Even though many small insects in English might be referred to generally as a bug, Spanish here is using the specific word pulgón.

The accent mark shows where the stress goes: pul-GÓN.

Without the accent, normal Spanish stress rules would make you stress it differently. The written accent tells you the final syllable is stressed.

So it is pronounced roughly:

• pul-GON

This is also useful because many Spanish learners might otherwise guess PUL-gon, which would be incorrect.

In most of Spain today, ll is usually pronounced the same as y. This is called yeísmo.

So tallo is commonly pronounced roughly like:

• TAH-yo

However, pronunciation varies by region. In older or more conservative pronunciation, ll could sound slightly different, but for most learners of modern Spain Spanish, pronouncing it like y is normal and understood.

Tallo means stem or stalk, especially the main supporting part of a plant that is not a tree trunk.

So:

• el tallo de una flor = the stem of a flower
• el tallo de una planta = the stem of a plant

It is not the same as tronco:

• tronco = trunk, especially of a tree
• tallo = stem/stalk

So in this sentence, en el tallo means the aphid was on the stem of the plant.

Spanish en often covers both in and on, depending on context.

So:

• en la mesa = on the table
• en la caja = in the box
• en el tallo = on the stem

Even though English distinguishes in and on, Spanish often uses en for both.

En el tallo is the most natural choice here.

• en is the normal preposition for location in many everyday cases
• sobre can mean on top of, over, or about, and sometimes sounds more literal or more marked

With something attached to or situated on part of a plant, en el tallo sounds natural and idiomatic.

So while sobre is possible in some contexts, en el tallo is what a learner should expect and use here.

The personal a is usually used before specific human beings, and sometimes pets or personified animals.

Examples:

• Vi a María = I saw María
• Vi a mi perro = I saw my dog

But with a non-person object like un pulgón, you normally do not use a:

• Vi un pulgón = I saw an aphid

So Vi a un pulgón would not be the normal form here.

Yes, but the meaning changes slightly.

• Vi un pulgón en el tallo = I saw an aphid on the stem
  
  • suggests a particular stem, probably one already known from context
• Vi un pulgón en un tallo = I saw an aphid on a stem
  
  • less specific
• Vi un pulgón sobre el tallo = I saw an aphid on top of / on the stem
  
  • possible, but less neutral than en

So el tallo is definite because the speaker likely means the stem of the plant being discussed or observed.

The usual and most natural order is:

• Vi un pulgón en el tallo.

This follows a common pattern:

• verb + object + location

You could change the order for emphasis, for example:

• En el tallo vi un pulgón.

That is still correct, but it sounds more marked, as if emphasizing where the aphid was.

For a learner, the original order is the safest and most natural one.

It is close, but it should be pronounced in a Spanish way, with a short, clean vowel:

• vi ≈ bee/v sound + ee

In most varieties of Spanish, b and v are pronounced very similarly, so vi does not sound like the strong English v in all contexts.

The important part for learners is:

• one syllable
• a clear ee sound
• no diphthong

So just say it simply as vi.

Yes. Pulgón is the usual word for aphid, the small plant-eating insect often found on stems and leaves.

This matters because Spanish is being quite precise here:

• insecto = insect
• bicho = bug/creepy-crawly thing, informal and broader
• pulgón = aphid

So the sentence is not just saying I saw a bug on the stem; it is specifically an aphid.