What your baby is doing: Babies listen from day one. They learn to associate sounds with their sources, like barking with the family dog. Her first communication will be crying, but she'll soon begin to use her tongue, lips, and palate to make gurgles and long vowel sounds like "oo," "aa," and "ee"—precursors to baby's first words.
What your baby can understand: Babies as young as 4 weeks can distinguish between similar syllables like "ma" and "na." As young as 2 months, they begin to associate certain sounds with certain lip movements.
What your baby is doing: Sighs give way to babbling. You'll hear back-of-the-tongue consonant sounds, such as g and k, and lip sounds m, w, p, and b. He focuses on familiar words, his own name, or "mommy" and "daddy" as clues to help break up sentences.
What your baby can understand: At 4-1/2 months, he may recognize his name, but only as an important word, such as "Hi!" or "Bye!" It's not until 6 months, at the earliest, that he'll realize his name actually refers to himself.
What your baby is doing: Her babbling will begin to sound more like words. She'll intentionally repeat sounds (like "gaga") over and over. At about 9 months, she'll start to understand gestures, pointing and grunting to indicate her wants. At about 10 months, she'll gain more control and begin combining sounds, even using her own invented words. The first real word often appears around 12 months. Common first words may be greetings ("hi" or "bye-bye"). Or they might be very concrete: people ("ma, ma" or "da, da"), pets ("doggy" or "kitty"), or food ("cookie," "juice," or "milk").
What your baby can understand: Your baby is slowly beginning to recognize and comprehend a few familiar words, things like names and everyday objects such as "bottle" or "crib." She keys in on intonation, realizing that a sharp tone often means "No!" or "Stop!"
What your toddler is doing: As soon as your baby gets out that first word, he'll try for more. Vocabulary builds slowly at first, just a few words a month. Kids seem to prefer nouns at first, then gradually add verbs and adjectives. He'll experiment with one-word questions, like "Cookie?" for "May I have a cookie?" and delight in saying "No!"
What your toddler can understand: He should understand the first rudiments of grammar, such as the difference between "The dog bit the man." and "The man bit the dog." He should grasp simple one-step instructions ("Get the ball") and understand many more words than he can say.
As infants learn to communicate, they progress at very different rates. Your baby may lag behind at some points, but as long as she produces syllables with consonants (such as "ba" or "da") by 10 months and doesn't suddenly lose the ability to babble once she's gained it, experts say there's no need to worry.
What your toddler is doing: Though linguists aren't sure why, toddlers at about 19 to 20 months have a "language explosion." After months of slow progress, they suddenly start to learn words at a ferocious rate, as many as nine a day.
This explosion of words leads to the exhausting "Wassat?" stage. By the end of the second year, your toddler will be stringing two, or even four, words together in sentences. This is also an age of cute mistakes, as kids overextend and "under-extend" concepts. For instance, your child may learn that the round toy is a "ball," figure all round things must be balls and point to the full moon, and chirp, "Ball!"
What your toddler can understand: Your baby will slowly begin to understand the idea of verbs. Fully aware that you are her key to language, she will watch and listen to you, absorbing everything you say and do.
What your toddler is doing: Refining what he's learned so far. He adds "When? Why? Where?" to "Wassat?" He begins to add complex ideas, learning that "no" can mean "not" or "don't" or "it's all gone." Late in the year, he may begin to use more abstract verbs like "think" and "know." As he gains control of the tip of his tongue while speaking, he begins to manage sounds like ph, th, and r.
What your toddler can understand: He will begin to understand tense, plurals, and suffixes such as "ing" and "ly." Late in this third year, your child should be speaking in two-word sentences, such as "Drink milk" or "Play ball."
Things you can do to help: Rhyming games help build awareness of language sounds. If he makes a mistake, repeat the sentence back correctly instead of drawing attention to the error. For instance, if he says, "I goed playground." You can say back, "You went to the playground? Great!"
What to watch: Kids' thoughts may go beyond their ability to form words. If stuttering, or some other problem, like a lisp, concerns you, consult your pediatrician.
What your toddler is doing: Conveying whole thoughts by employing just a few words, like saying "Mommy no socks" for "Mommy isn't wearing any socks today." Later in the year, she'll speak in longer sentences, putting several thoughts together to tell a story in about 300 words.
What your toddler can understand: She should be able to follow a storyline and remember ideas from it. She'll begin to enjoy nonsense phrases.
Things you can do to help: Read to her from storybooks with more of a narrative. Kids need more assistance than we do for conversation. Take a look at her preschool class list, and start making stuff up. Was Mary in school today? Add something silly, like "Was she wearing that hat with the fruit on it again?" See what happens—you may find out what's going on.
What your toddler is doing: By this age, your child should be having extensive conversations with adults; using adjectives in detailed sentences; telling knock-knock jokes; and asking questions with proper intonation. Before he turns 6, he'll likely have an expressive vocabulary of around 2,500 words.
What your toddler can understand: About 14,000 words. He'll also be able to express complicated thoughts like fears and dreams, say "thank you", and use words to elicit reactions from others.
Things you can do to help: Don't criticize any missteps in articulation or speech. Instead, repeat his statements back to him with the correct pronunciation or word usage. Give your child lots of praise for his efforts.
What to watch for: Too much screen time. The American Academy of Pediatrics recommends that children 2 and older view no more than two hours of quality programming a day. Kids need interaction and response to learn language. Most TV shows don't interact, and computer games aren't responsive to a child's ideas.