Numbers in Hebrew are rather confusing topic, especially for English speakers. In English we have two types of numbers: cardinal and ordinal: one and first, two and second, seventeen and seventeenth, and so on.. That's pretty simple and straightforward (or at least they seem to be if English is your first language.) They are all gender neutral, and the ordinal numbers are derived from their cardinal form with a simple addition of -th suffix.

Let's start using this as basis for Hebrew. First of all, we need to add another "dimension": grammatic gender. Numbers in Hebrew are, strictly speaking, nouns, and they can be masculine or feminine:

English		Hebrew (fem.)		Hebrew (masc.)
one	1	אַחַת		אֶחָד
two	2	שְׁתַיִם		שְׁנַיִם
three	3	שָׁלוֹשׁ		שְׁלוֹשָה
four	4	אַרְבַּע		אַרְבָּעָה
five	5	חָמֵש		חֲמִשָּׁה
six	6	שֵׁשׁ		שִׁשָּׁה
seven	7	שֶׁבַע		שִׁבְעָה
eight	8	שְמוֹנֶה		שְׁמוֹנָה
nine	9	תֵּשַׁע		תִּשְׁעָה
ten	10	עֶשֶׂר		עֲשָׂרָה

Worth to mention, that the "default" form for nouns is feminine. When you count, it's אחת, שתים, שלוש, ארבע, etc,

Also, note the following:

• For most of numerals you say just ארבעה בנים or שלושה אמהות - which is the way you're used to say it: "four sons", "three mothers".
• For ONE, you put the numeral after the object: בֵּן אחד (one son), בַּת אחת (one daughter.)
• For TWO, the stand-alone number changes into its combinational form (a.k.a. Nismakh): שְׁנֵי בָּנִים,   שְׁתֵי בָּנוֹת (two sons, two daughters.)

Zero is אֶפֶס and it doesn't have masculine or feminine form. Nothing is nothing.

Similar to English, there are ordinal numbers in Hebrew. The first one is "first", and it does not look like "one": it's רִאשׁוֹן rather than אַחַת. You may notice that the word רִאשׁוֹן is derived from רֹאשׁ (head), that's why it doesn't look like אַחַת.

Here are all the ordinal numbers 1 to 10 -- you may note that they look pretty much like ordinary adjectives, derived from the cardinal numbers (which are nouns.)

English		Hebrew (fem.)		Hebrew (masc.)
first	1	רִאשׁוֹנָה		רִאשׁוֹן
second	2	שְׁנִיָּה		שֵׁנִי
third	3	שְׁלִישִׁית		שְׁלִישִי
fourth	4	רְבִיעִית		רְבִיעִי
fifth	5	חֲמִשִּית		חֲמִשִּׁי
sixth	6	שִׁשִּׁית		שִׁשָּׁי
seventh	7	שְׁבִיעִית		שְׁבִיעָי
eighth	8	שְמִינִית		שְׁמִינִי
ninth	9	תְּשִׁיעִית		תְּשִׁיעִי
tenth	10	עֲשִׂירִית		עֲשִׂירִי

As you can see, ordinal numbers from 2nd to 10th are derived directly from the cardinal numbers.

I would recommend to memorize this part before going any forward. Both cardinal and ordinal numbers are essential parts of the vocabulary in any language; but we also can find many ancient and modern Israeli names which include numbers. Let's read some of them, which is also a good exercise for you:

Hebrew (fem.)		Translation:
בְּאֵר-שֶׁבַע		"Well of (the) Seven" well -- בְּאֵר
קִרְיַת-אַרְבַּע		"Town of (the) Four" town  -  קִרְיָה (combinational form) -  קִרְיַת
קִרְיַת-שְׁמוֹנָה
רִאשׁוֹן לְצִיּוֹן		"First in Zion" (a city near Tel-Aviv) in Zion -- לְצִיּוֹן
שְׁנַיִם - מִי יוֹדֵעַ?		Two - Who Knows? (from the Haggadah) who knows - מִי יוֹדֵעַ
שִׁשָּׁה סִדְרֵי מִשְׁנָה		The six "orders" (books) of Mishnah
וּבַיּוֹם הַשְׁבִיעִי שָבַת וַיִּנָּפָש		And in the 7th day stopped working and rested. and in the day - וּבַיּוֹם the - הַ stopped working -  שָבַת and rested -  יִנָּפָש
הַקּוֹנְגְרֶס הַצִּיּוֹנִי הָעוֹלָמִי הָרִאשׁוֹן		First World Zionist Congress (1897)
אַרְבָעָה בָנִים		"The four sons" (from Haggadah) sons - בָּנִים
שְׁלוֹשָה אִמָּהוֹת		The Four Mothers (mothers -- אִמָּהוֹת)
כְּבִישׁ אַרְבַע		Route - כְּבִישׁ (Route 4, the "Coast Route")
כְּבִישׁ מִסְפַּר שֵׁשׁ		number - מִסְפָּר (Route number 6)

Next time reading your prayer books or Israeli tourist guide, you will probably start to understand the meaning of some names. :)

Look on the phrase וביום השביעי שבת וינפש. It's a perfect example of ordinal numeral: it's clearly an adjective, and it behaves like one. It's getting the "article" (ha- similar although not equivalent to English the), literally: וביום השביעי -- "and in the day the seventh", and it has the same gender (masculine) as the word יום (day.)

Remark about modern spoken Hebrew:

Also, the good news is, when you are talking about something abstract, you might simplifying the grammar too: "שלוש דולר" or "עשר שקל" rather than "שלושה דולרים" or "עשרה שקלים" (which is "more correct", as your grammar teacher would probably say.) It's hard to formalize it as a rule though. It works for sure with money, in most cases with measurement units too, and in some other cases. I'm not sure whether the same simplification existed in classic Hebrew though.

As an exercise, go and read the Passover song "One - who knows?" :) You will find a lot of numericals there.

Good luck, and see you in the next lesson about numerals!