What kind of helping verbs that always keep the same form no matter what subject is?

You're asking about a special category of helping verbs that don't change form based on the subject. This actually doesn't exist.

Here's why:

* Helping verbs (also called auxiliary verbs) are used in conjunction with main verbs to express different tenses, moods, and voices. They help the main verb convey additional information.

* Some helping verbs do change form depending on the subject:

* Be (am, is, are, was, were)

* Have (has, have, had)

* Do (does, do, did)

Example:

* *She is writing a letter.* ("Is" changes to "are" if the subject is plural, like "They")

Let's look at what you might have in mind:

* Modal verbs: These are a type of helping verb that *always* stay the same, regardless of the subject. They convey things like possibility, necessity, and permission.

* Examples: can, could, may, might, must, should, will, would

Example:

* *He can swim.* ("Can" doesn't change for "She" or "They")

In short, while modal verbs always maintain their form, regular helping verbs (like forms of "be," "have," and "do") change their form according to the subject.