Я посмотрела на этикетку кроссовок и поняла, что взяла не тот размер.

I looked at the label on the sneakers and realized that I had taken the wrong size.

Breakdown of Я посмотрела на этикетку кроссовок и поняла, что взяла не тот размер.

• я: I
• на: at
• и: and
• не: not

Questions & Answers about Я посмотрела на этикетку кроссовок и поняла, что взяла не тот размер.

Why do the verbs end in -ла: посмотрела, поняла, взяла?

Because the speaker is female.

In the past tense, Russian verbs agree with gender and number:

• masculine: посмотрел, понял, взял
• feminine: посмотрела, поняла, взяла
• neuter: посмотрело
• plural: посмотрели

So this sentence sounds as if a woman is speaking. If a man said it, it would be:

Я посмотрел на этикетку кроссовок и понял, что взял не тот размер.

Why are all the verbs perfective: посмотрела, поняла, взяла?

They describe completed actions or single events.

• посмотрела = took a look
• поняла = realized / came to understand
• взяла = took / picked

Perfective aspect is very natural here because the sentence tells a sequence of finished events:

1. she looked,
2. then realized,
3. that she had chosen the wrong size.

If you used imperfective forms, the meaning would shift:

• смотрела = was looking / looked repeatedly
• понимала = understood / was understanding
• брала = was taking / used to take

Those would not fit this one-time discovery as well.