The word "equal" has three parts of speech: noun, verb, and adjective.

1. four plus two equals six

In your first example sentence, "equal" is being used as a verb in the third person singular form: equals.

2. four plus two is equal to six

In your second example, "equal" is being used as an adjective, and so it needs to employ a linking verb (in this case, "be" or "is") before it. The "to" after it is a preposition.

3. four plus two is six

Finally, here "is" is again a linking verb in an S + LV + C pattern (Subject + Linking Verb + Complement).

The word "equal" has three parts of speech: noun, verb, and adjective.

1. four plus two equals six

   In your first example sentence, "equal" is being used as a verb in the third person singular form: equals.

2. four plus two is equal to six

   In your second example, "equal" is being used as an adjective, and so it needs to employ a linking verb (in this case, "be" or "is") before it. The "to" after it is a preposition.

3. four plus two is six

   Finally, here "is" is again a linking verb in an S + LV + C pattern (Subject + Linking Verb + Complement).

The word "equal" has three parts of speech: noun, verb, and adjective.

four plus two equals six

In your first example sentence, "equal" is being used as a verb in the third person singular form: equals.

four plus two is equal to six 

In your second example, "equal" is being used as an adjective, and so it needs to employ a linking verb (in this case, "be" or "is") before it. The "to" after it is a preposition.

four plus two is six

Finally, here "is" is again a linking verb in an S + LV + C pattern (Subject + Linking Verb + Complement).

The word "equal" has three parts of speech: noun, verb, and adjective.

1. four plus two equals six

In your first example sentence, "equal" is being used as a verb in the third person singular form: equals.

2. four plus two is equal to six

In your second example, "equal" is being used as an adjective, and so it needs to employ a linking verb (in this case, "be" or "is") before it. The "to" after it is a preposition.

3. four plus two is six

Finally, here "is" is again a linking verb in an S + LV + C pattern (Subject + Linking Verb + Complement).