This worksheet helps young learners master indefinite articles by matching 'a' or 'an' to words like 'owl,' 'ambulance,' and 'igloo.'

To solve this problem, we need to determine whether to use "a" or "an" before each noun. The rule is:

- Use "a" before words that start with a consonant sound.
- Use "an" before words that start with a vowel sound.

Let's go through each word and apply this rule:

First Row:

1. butterfly: Starts with a consonant sound ("b"). → a butterfly
2. alien: Starts with a consonant sound ("a" is pronounced like "ay"). → an alien
3. tree: Starts with a consonant sound ("t"). → a tree
4. key: Starts with a consonant sound ("k"). → a key

Second Row:

5. underwear: Starts with a consonant sound ("u" is pronounced like "yoo"). → a underwear
6. guitar: Starts with a consonant sound ("g"). → a guitar
7. caterpillar: Starts with a consonant sound ("c" is pronounced like "k"). → a caterpillar
8. monkey: Starts with a consonant sound ("m"). → a monkey

Third Row:

9. dinosaur: Starts with a consonant sound ("d"). → a dinosaur
10. baby: Starts with a consonant sound ("b"). → a baby
11. ambulance: Starts with a consonant sound ("a" is pronounced like "uh"). → an ambulance
12. ice cream: Starts with a vowel sound ("i"). → an ice cream

Fourth Row:

13. owl: Starts with a vowel sound ("o"). → an owl
14. zip: Starts with a consonant sound ("z"). → a zip
15. elephant: Starts with a consonant sound ("e" is pronounced like "ee"). → an elephant
16. heart: Starts with a consonant sound ("h"). → a heart

Fifth Row:

17. airplane: Starts with a consonant sound ("a" is pronounced like "ay"). → an airplane
18. castle: Starts with a consonant sound ("c" is pronounced like "k"). → a castle
19. igloo: Starts with a vowel sound ("i"). → an igloo
20. spider: Starts with a consonant sound ("s"). → a spider

Final Answer:

```
a butterfly an alien a tree a key
a underwear a guitar a caterpillar a monkey
a dinosaur a baby an ambulance an ice cream
an owl a zip an elephant a heart
an airplane a castle an igloo a spider
```

Boxed Answer:
\boxed{
\begin{array}{llll}
\text{a butterfly} & \text{an alien} & \text{a tree} & \text{a key} \\
\text{a underwear} & \text{a guitar} & \text{a caterpillar} & \text{a monkey} \\
\text{a dinosaur} & \text{a baby} & \text{an ambulance} & \text{an ice cream} \\
\text{an owl} & \text{a zip} & \text{an elephant} & \text{a heart} \\
\text{an airplane} & \text{a castle} & \text{an igloo} & \text{a spider} \\
\end{array}
}