Let’s untangle the beautiful, logical, and mind-bending world of bí and is.

Meet ‘Bí’: The Verb of State and Action

Let’s start with the more familiar of the two. Bí is what linguists call the “substantive verb.” A simpler way to think of it is as the verb of state, location, and condition. If you want to say how something is, where it is, or what it is currently doing, you use bí.

Crucially, bí is a “normal” verb. It conjugates for person and tense just like you’d expect. Its present tense form is tá, which you’ll see everywhere.

Consider these examples:

• Location: Tá mé anseo. (I am here.)
• Temporary Condition: Tá an lá fuar. (The day is cold.)
• Feeling/Emotion: Tá siad tuirseach. (They are tired.)
• Ongoing Action (Present Progressive): Tá sé ag léamh. (He is reading.)

Notice a pattern? Bí/Tá is used to connect a subject to an adjective (fuar, tuirseach), a prepositional phrase (anseo), or an ongoing action (ag léamh). It describes the subject’s current situation or attributes. It’s dynamic and descriptive.

The Conjugating Verb

Because bí is a regular (albeit irregular!) verb, it has a full range of forms. You can talk about the past, present, and future with it:

• Present: Tá mé – I am
• Past: Bhí mé – I was
• Future: Beidh mé – I will be

This ability to change for time and person makes bí the workhorse verb for describing reality as it unfolds.

Unmasking ‘Is’: The Copula of Identity

Now we enter stranger territory. The second ‘to be’ is is, known as the “copula.” The word “copula” just means “linker”, and that’s exactly what is does. It works like a grammatical equals sign (=). Its job isn’t to describe a state, but to make a statement of identity, classification, or definition.

Is is a profoundly weird verb. It’s “defective”, meaning it has very few forms. It doesn’t conjugate for person at all. In the present tense, it’s just… is. It links two nouns or a noun and a pronoun.

Here’s how it works:

• Classification: Is dochtúir í. (She is a doctor.)
• Definition: Is ainmhí é an capall. (The horse is an animal.)
• Identity/Name: Is mise Siobhán. (I am Siobhán.)

The sentence structure here is a dead giveaway and a major hurdle for learners. The formula is typically: Is + [DEFINITION/NOUN] + [SUBJECT/PRONOUN]. Notice how the verb comes first, followed by the definition (“doctor”), and then the person (“she”). This is completely different from the Tá sí tuirseach structure.

The past tense of the copula is ba. So, “She was a doctor” is Ba dhochtúir í.

The Golden Rule: Adjectives vs. Nouns

So, what’s the secret? How do you decide between them? Forget “temporary vs. permanent.” The real dividing line is grammatical, and it’s surprisingly simple:

You use bí with adjectives. You use is with nouns.

This is the core principle that governs everything. Let’s see it in action.

If you want to say “He is tall”, ‘tall’ (ard) is an adjective. So, you use bí:

Tá sé ard. (He is tall.)

But if you want to say “He is a tall man”, ‘a tall man’ (fear ard) is a noun phrase. So, you must use is:

Is fear ard é. (He is a tall man.)

This distinction is absolute. You cannot say *Is sé ard*. It’s grammatically nonsensical, like saying “Equals he tall.” Likewise, you can’t say *Tá sé fear ard*, which would imply that being a tall man is a temporary state he’s in, like being tired or cold.

So, Why the ‘Temporary vs. Permanent’ Confusion?

The old “temporary vs. permanent” rule isn’t wrong, exactly—it’s just a shadow of the real rule. States described by adjectives (tired, happy, cold, here) are often temporary. Classifications made with nouns (a doctor, an Irish person, a woman, a fool) are generally seen as more stable identities. The linguistic rule (adjective vs. noun) creates a philosophical side effect that happens to look a lot like a temporary/permanent divide. But the grammatical rule is what’s really in the driver’s seat.

The Copula’s Other Job: Emphasis

The copula is has another vital role: creating “cleft sentences.” This sounds technical, but it’s just a way of adding emphasis, much like saying “It is/was [X] that did [Y]” in English.

Look at this simple sentence using a normal verb:

Bhris an páiste an fhuinneog. (The child broke the window.)

Now, what if you want to stress that it was the child (and not someone else) who broke it? You use the copula to pull the subject to the front:

Is é an páiste a bhris an fhuinneog. (It is the child who broke the window.)

This structure is incredibly common in Irish and is used for everything from answering questions to telling stories with dramatic flair. It’s also how you say you like something. You don’t say “I like coffee.” You say:

Is maith liom caife. (Literally: “Is good with-me coffee.”)

Here, the copula establishes the fact: “goodness exists with me in relation to coffee.”

A Different Way of Seeing

Learning the two ‘to be’s of Irish isn’t just a grammar lesson; it’s a glimpse into a different linguistic worldview. The language forces you to decide: are you simply describing a feature of something, or are you defining its very essence? Are you talking about a passing state or a classification?

It’s a distinction that, once mastered, feels incredibly precise and logical. It separates the fluid, changing world of descriptions (tá) from the solid, categorical world of definitions (is). And in that division lies a key piece of the unique beauty of the Irish language.