Lines 9-14 of the first poem of Sir Philip Sidney’s sonnet sequence titled Astrophil and Stella might be explained as follows:

But words came halting forth, wanting Invention's stay,

In spite of all my efforts to find the right language with which to make Stella return my affections, my words walked as if they were crippled, lacking the support of Invention.  “Invention” was a key element of classical and Renaissance rhetoric.  It meant the power to invent the words one needed – literally, the power to “come upon” them.

Invention, Nature's child, fled stepdame Study's blows,

Invention, which is referred to here as if it were a very young person, is the offspring of Nature.  In other words, finding the right language when one is trying to write depends on natural talent. Such talent cannot be entirely taught. Thus, Invention, which is like a child, flees from the punishment inflicted by a personified Study.  Study is here compared to a harsh stepmother (or schoolmistress or tutor) who uses beatings to make sure that children learn their lessons.

And other's feet still seem'd but strangers in my way.

The speaker had little success in finding appropriate language by studying what others wrote.  The “feet” (that is, the metrical feet used in poetry, such as an iamb or trochee) stilled seemed foreign to him. In other words, he was still unable to write poetry, even though he had tried to imitate other writers and other writings.

Thus great with child to speak and helpless in my throes,

Although he had much that he wanted to say, he was unable to say it.  He therefore felt like a woman suffering enormous pains as she strains to give birth to a baby that will not come.

Biting my truand pen, beating myself for spite,

Astrophil therefore bit his disobedient pen, probably out of frustration. He even hit himself out of anger.

“Fool,” said my Muse to me, “look in thy heart and write.”

Just when he was feeling extremely frustrated, he heard the voice of his muse, a supernatural goddess who inspires poets.  (The nine muses of classical mythological were believed to inspire excellence in a wide variety of art forms.) She called him a fool (an unintelligent, ridiculous person). She told him that if he wanted to write successfully, he should examine and draw on his own emotions, not study the writings of others.