אל תשתמשי בפטיש הגדול הזה; המסמר קטן מדי בשביל הקיר הזה.

Because the sentence is speaking to one woman.

• תשתמש = you will use / use! when speaking to one man
• תשתמשי = the same, but to one woman

So אל תשתמשי means don’t use said to a female listener.

If you were talking to a man, it would be:

אל תשתמש בפטיש הגדול הזה

In Modern Hebrew, a negative command is usually made with:

אל + future-form verb

So:

• תשתמשי = you will use / command form in this pattern
• אל תשתמשי = don’t use

This is different from לא, which usually negates statements rather than commands.

• לא תשתמשי = you will not use or you don’t use in context
• אל תשתמשי = don’t use!

The dictionary form is להשתמש, meaning to use.

This verb belongs to the pattern התפעל and is commonly learned as:

• infinitive: להשתמש
• present masculine singular: משתמש
• present feminine singular: משתמשת
• future / command-related form here: תשתמשי

A useful chunk to memorize is:

להשתמש ב־ = to use

Because the verb להשתמש normally takes the preposition ב־.

So Hebrew says, literally, something like to use with / to make use of something.

Examples:

• להשתמש בפטיש = to use a hammer
• להשתמש במחשב = to use a computer
• להשתמש במילה הזאת = to use this word

So after להשתמש, the ב־ is standard.

Because the preposition ב־ combines with the definite article ה־.

So:

• ב + הפטיש becomes בפטיש

In fully pointed Hebrew, this reflects a merged form, but in normal unpointed spelling you usually just see בפטיש.

So בפטיש הגדול הזה means with this big hammer or using this big hammer, not in hammer.

The same kind of thing happens with other prefixes too, such as ל־ and כ־.

Because in Hebrew, adjectives usually come after the noun, and the demonstrative הזה / הזאת / האלה also usually comes after the noun phrase.

So the order is:

noun + adjective + demonstrative

Examples:

• הפטיש הגדול הזה = this big hammer
• הקיר הזה = this wall
• הספר החדש הזה = this new book

That is normal Hebrew word order.

Because they do different jobs, but both follow the noun in this kind of phrase.

• הגדול describes the noun: big
• הזה points to it: this

So:

• פטיש גדול = a big hammer
• הפטיש הגדול = the big hammer
• הפטיש הגדול הזה = this big hammer

Hebrew stacks them after the noun rather than before it, unlike English.

מדי means too, as in excessively.

It comes after the adjective:

• קטן מדי = too small
• גדול מדי = too big
• מהיר מדי = too fast

So:

המסמר קטן מדי = the nail is too small

Here בשביל means for in the sense of for the purposes of, for, or suitable for.

So:

קטן מדי בשביל הקיר הזה = too small for this wall

In everyday Hebrew, בשביל is very common and can mean:

• for
• for the sake of
• sometimes because of, depending on context

In this sentence, it is basically saying the nail is not suitable for this wall.

Because the nouns they describe are masculine singular:

• פטיש = masculine singular
• מסמר = masculine singular
• קיר = masculine singular

Hebrew adjectives and demonstratives must agree with the noun in gender and number.

So:

• פטיש גדול = a big hammer
• מסמר קטן = a small nail
• הקיר הזה = this wall

If the noun were feminine, the forms would change:

• גדולה instead of גדול
• קטנה instead of קטן
• הזאת instead of הזה

Only the command verb changes.

To one man:

אל תשתמש בפטיש הגדול הזה; המסמר קטן מדי בשביל הקיר הזה.

To more than one person:

אל תשתמשו בפטיש הגדול הזה; המסמר קטן מדי בשביל הקיר הזה.

So the key forms are:

• אל תשתמש = don’t use, to one male
• אל תשתמשי = don’t use, to one female
• אל תשתמשו = don’t use, to plural listeners